General Questions
Tell us about yourself/ your background.
What are the three major characteristics that you bring to the job
market?
What motivates you to do a good job?
What two or three things are most important to you at work?
What qualities do you think are essential to be successful in this kind
of work?
What courses did you attend? What job certifications do you hold?
What subjects/courses did you excel in? Why?
What subjects/courses gave you trouble? Why?
How does your previous work
experience prepare you for this position?
How do you define 'success'?
What has been your most significant accomplishment to date?
Describe a challenge you encountered and how you dealt with it.
Describe a failure and how you dealt with it.
Describe the 'ideal' job... the 'ideal' supervisor.
What leadership roles have you held?
What prejudices do you hold?
What do you like to do in your spare time?
What are your career goals (a) 3 years from now; (b) 10 years from now?
How does this position match your career goals?
What have you done in the past year to improve yourself?
In what areas do you feel you need further education and training to be
successful?
What do you know about our company?
Why do you want to work for this company. Why should we hire you?
Where do you see yourself fitting in to this organization ...initially?
...in 5 years?
Why are you looking for a new job?
How do you feel about re-locating?
Are you willing to travel?
What are your salary requirements?
When would you be available to start if you were selected?
Here are few interview questions with
answers found on the internet. As I don't have time to format these questions
to wiki I am just posting them hoping someone to format them.
[edit] Oracle Interview Questions
1. Explain the difference between a
hot backup and a cold backup and the benefits associated with each.
A hot backup is basically taking a backup of the database while it is
still up and running and it must be in archive log mode. A cold backup is
taking a backup of the database while it is shut down and does not require
being in archive log mode. The benefit of taking a hot backup is that the
database is still available for use while the backup is occurring and you can
recover the database to any point in time. The benefit of taking a cold backup
is that it is typically easier to administer the backup and recovery process.
In addition, since you are taking cold backups the database does not require
being in archive log mode and thus there will be a slight performance gain as
the database is not cutting archive logs to disk.
2. You have just had to restore from
backup and do not have any control files. How would you go about bringing up
this database?
I would create a text based backup control file, stipulating where on
disk all the data files were and then issue the recover command with the using
backup control file clause.
3. How do you switch from an init.ora
file to a spfile?
Issue the create spfile from pfile command.
4. Explain the difference between a
data block, an extent and a segment.
A data block is the smallest unit of logical storage for a database
object. As objects grow they take chunks of additional storage that are
composed of contiguous data blocks. These groupings of contiguous data blocks
are called extents. All the extents that an object takes when grouped together
are considered the segment of the database object.
5. Give two examples of how you might
determine the structure of the table DEPT.
Use the describe command or use the dbms_metadata.get_ddl package.
6. Where would you look for errors
from the database engine?
In the alert log.
7. Compare and contrast TRUNCATE and
DELETE for a table.
Both the truncate and delete command have the desired outcome of getting
rid of all the rows in a table. The difference between the two is that the
truncate command is a DDL operation and just moves the high water mark and
produces few rollback data. The delete command, on the other hand, is a DML
operation, which will produce rollback data and thus take longer to complete.
8. Give the reasoning behind using an
index.
Faster access to data blocks in a table.
9. Give the two types of tables involved
in producing a star schema and the type of data they hold.
Fact tables and dimension tables. A fact table contains measurements
while dimension tables will contain data that will help describe the fact
tables.
10. What type of index should you use
on a fact table?
A Bitmap index.
11. Give some examples of the types of
database contraints you may find in Oracle and indicate their purpose.
A Primary or Unique Key can be used to enforce uniqueness on one or more
columns.
A Referential Integrity Contraint can be used to enforce a Foreign Key
relationship between two tables.
A Not Null constraint - to ensure a value is entered in a column
A Value Constraint - to check a column value against a specific set of
values.
12. A table is classified as a parent
table and you want to drop and re-create it. How would you do this without
affecting the children tables?
Disable the foreign key constraint to the parent, drop the table,
re-create the table, enable the foreign key constraint.
13. Explain the difference between
ARCHIVELOG mode and NOARCHIVELOG mode and the benefits and disadvantages to
each.
ARCHIVELOG mode is a mode that you can put the database in for creating
a backup of all transactions that have occurred in the database so that you can
recover to any point in time. NOARCHIVELOG mode is basically the absence of
ARCHIVELOG mode and has the disadvantage of not being able to recover to any
point in time. NOARCHIVELOG mode does have the advantage of not having to write
transactions to an archive log and thus increases the performance of the
database slightly.
14. What command would you use to
create a backup control file?
Alter database backup control file to trace.
15. Give the stages of instance
startup to a usable state where normal users may access it.
STARTUP NOMOUNT - Instance startup
STARTUP MOUNT - The database is mounted
STARTUP OPEN - The database is opened
16. What column differentiates the V$
views to the GV$ views and how?
The INST_ID column which indicates the instance in a RAC environment the
information came from.
17. How would you go about generating
an EXPLAIN plan?
Create a plan table with utlxplan.sql.
Use the explain plan set statement_id = 'tst1' into plan_table for a SQL
statement
Look at the explain plan with utlxplp.sql or utlxpls.sql
18. How would you go about increasing
the buffer cache hit ratio?
Use the buffer cache advisory over a given workload and then query the
v$db_cache_advice table. If a change was necessary then I would use the alter
system set db_cache_size command.
19. Explain an ORA-01555.
You get this error when you get a snapshot too old within rollback. It
can usually be solved by increasing the undo retention or increasing the size
of rollbacks. You should also look at the logic involved in the application
getting the error message.
20. Explain the difference between
$ORACLE_HOME and $ORACLE_BASE.
ORACLE_BASE is the root directory for oracle. ORACLE_HOME located
beneath ORACLE_BASE is where the oracle products reside.
[edit] Oracle Interview Questions
1. How would you determine the time
zone under which a database was operating?
SELECT dbtimezone FROM DUAL;
2. Explain the use of setting
GLOBAL_NAMES equal to TRUE.
It ensure the use of consistent naming conventions for databases and
links in a networked environment.
3. What command would you use to
encrypt a PL/SQL application?
WRAP
4. Explain the difference between a
FUNCTION, PROCEDURE and PACKAGE.
They are all named PL/SQL blocks.
Function must return a value. Can be called inside a query.
Procedure may or may not return value.
Package is the collection of functions, procedures, variables which can
be logically grouped together.
5. Explain the use of table functions.
6. Name three advisory statistics you
can collect.
7. Where in the Oracle directory tree
structure are audit traces placed?
8. Explain materialized views and how
they are used.
9. When a user process fails, what
background process cleans up after it?
PMON
10. What background process refreshes
materialized views?
Job Queue Process (CJQ)
11. How would you determine what
sessions are connected and what resources they are waiting for?
v$session,v$session_wait
12. Describe what redo logs are.
13. How would you force a log switch?
alter system switch logfile;
14. Give two methods you could use to
determine what DDL changes have been made.
15. What does coalescing a tablespace
do?
Coalesce simply takes contigous free extents and makes them into a
single bigger free extent.
16. What is the difference between a
TEMPORARY tablespace and a PERMANENT tablespace?
TEMP tablespace gets cleared once the transaction is done where as
PERMANENT tablespace retails the data.
17. Name a tablespace automatically
created when you create a database.
SYSTEM
18. When creating a user, what
permissions must you grant to allow them to connect to the database?
Grant create session to username;
19. How do you add a data file to a
tablespace?
alter tablespace USERS add datafile '/ora01/oradata/users02.dbf' size
50M;
20. How do you resize a data file?
alter database datafile '/ora01/oradata/users02.dbf' resize 100M;
21. What view would you use to look at
the size of a data file?
dba_data_files
22. What view would you use to
determine free space in a tablespace?
dba_free_space
23. How would you determine who has
added a row to a table?
By implementing an INSERT trigger for logging details during each INSERT
operation on the table
24. How can you rebuild an index?
ALTER INDEX index_name REBUILD;
25. Explain what partitioning is and
what its benefit is.
A table partition is also a table segment, and by using partitioning
technique we can enhance performance of table access.
26. You have just compiled a PL/SQL
package but got errors, how would you view the errors?
show errors
27. How can you gather statistics on a
table?
exec dbms_stats.gather_table_stats
Also, remember to analyze all associated indexes on that table using
dbms_stats.gather_index_stats
28. How can you enable a trace for a
session?
alter session set sql_trace='TRUE';
29. What is the difference between the
SQL*Loader and IMPORT utilities?
SQL*LOADER loads external data which is in OS files to oracle database
tables while IMPORT utility imports data only which is exported by EXPORT
utility of oracle database.
30. Name two files used for network
connection to a database.
TNSNAMES.ORA and SQLNET.ORA
[edit] Oracle Interview Questions
1. Describe the difference between a
procedure, function and anonymous pl/sql block. Candidate should mention use of
DECLARE statement, a function must return a value while a procedure doesn't
have to.
2. What is a mutating table error and
how can you get around it? This happens with triggers. It occurs because the
trigger is trying to update a row it is currently using. The usual fix involves
either use of views or temporary tables so the database is selecting from one
while updating the other.
3. Describe the use of %ROWTYPE and
%TYPE in PL/SQL Expected
Answer: %ROWTYPE allows you to
associate a variable with an entire table row. The %TYPE associates a variable
with a single column type.
4. What packages (if any) has Oracle
provided for use by developers? Expected answer: Oracle provides the DBMS_
series of packages. There are many which developers should be aware of such as
DBMS_SQL, DBMS_PIPE, DBMS_TRANSACTION, DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT,
DBMS_JOB, DBMS_UTILITY, DBMS_DDL, UTL_FILE. If they can mention a few of these
and describe how they used them, even better. If they include the SQL routines
provided by Oracle, great, but not really what was asked.
5. Describe the use of PL/SQL tables
Expected
Answer: PL/SQL tables are scalar
arrays that can be referenced by a binary integer. They can be used to hold
values for use in later queries or calculations. In Oracle 8 they will be able
to be of the %ROWTYPE designation, or RECORD.
6. When is a declare statement needed
?
The DECLARE statement is used in
PL/SQL anonymous blocks such as with stand alone, non-stored PL/SQL procedures.
It must come first in a PL/SQL stand alone file if it is used.
7. In what order should a open/fetch/loop
set of commands in a PL/SQL block be implemented if you use the %NOTFOUND
cursor variable in the exit when statement? Why? Expected answer: OPEN then
FETCH then LOOP followed by the exit when. If not specified in this order will
result in the final return being done twice because of the way the %NOTFOUND is
handled by PL/SQL.
8. What are SQLCODE and SQLERRM and
why are they important for PL/SQL developers? Expected
Answer: SQLCODE returns the value of
the error number for the last error encountered. The SQLERRM returns the actual
error message for the last error encountered. They can be used in exception
handling to report, or, store in an error log table, the error that occurred in
the code. These are especially useful for the WHEN OTHERS exception.
9. How can you find within a PL/SQL
block, if a cursor is open? Expected answer: Use the %ISOPEN cursor status
variable.
10. How can you generate debugging
output from PL/SQL? Expected answer: Use the DBMS_OUTPUT package. Another
possible method is to just use the SHOW ERROR command, but this only shows
errors. The DBMS_OUTPUT package can be used to show intermediate results from
loops and the status of variables as the procedure is executed. The new package
UTL_FILE can also be used.
11. What are the types of triggers?
Expected Answer: There are 12 types of triggers in PL/SQL that consist of
combinations of the BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and ALL
key words: BEFORE ALL ROW INSERT AFTER ALL ROW INSERT BEFORE INSERT AFTER
INSERT etc.
[edit] Oracle Interview Questions
1. A tablespace has a table with 30
extents in it. Is this bad? Why or why not.
Multiple extents in and of themselves
aren?t bad. However if you also have chained rows this can hurt performance.
2. How do you set up tablespaces
during an Oracle installation?
You should always attempt to use the
Oracle Flexible Architecture standard or another partitioning scheme to ensure
proper separation of SYSTEM, ROLLBACK, REDO LOG, DATA, TEMPORARY and INDEX
segments.
3. You see multiple fragments in the
SYSTEM tablespace, what should you check first?
Ensure that users don?t have the
SYSTEM tablespace as their TEMPORARY or DEFAULT tablespace assignment by
checking the DBA_USERS view.
4. What are some indications that you
need to increase the SHARED_POOL_SIZE parameter?
Poor data dictionary or library cache
hit ratios, getting error ORA-04031. Another indication is steadily decreasing
performance with all other tuning parameters the same.
5. What is the general guideline for
sizing db_block_size and db_multi_block_read for an application that does many
full table scans?
Oracle almost always reads in 64k
chunks. The two should have a product equal to 64 or a multiple of 64.
6. What is the fastest query method
for a table
Fetch by rowid
7. Explain the use of TKPROF? What
initialization parameter should be turned on to get full TKPROF output?
The tkprof tool is a tuning tool used
to determine cpu and execution times for SQL statements. You use it by first
setting timed_statistics to true in the initialization file and then turning on
tracing for either the entire database via the sql_trace parameter or for the
session using the ALTER SESSION command. Once the trace file is generated you
run the tkprof tool against the trace file and then look at the output from the
tkprof tool. This can also be used to generate explain plan output.
8. When looking at v$sysstat you see
that sorts (disk) is high. Is this bad or good? If bad -How do you correct it?
If you get excessive disk sorts this
is bad. This indicates you need to tune the sort area parameters in the
initialization files. The major sort are parameter is the SORT_AREA_SIZe
parameter.
9. When should you increase copy latches?
What parameters control copy latches
When you get excessive contention for
the copy latches as shown by the "redo copy" latch hit ratio. You can
increase copy latches via the initialization parameter LOG_SIMULTANEOUS_COPIES
to twice the number of CPUs on your system.
10. Where can you get a list of all
initialization parameters for your instance? How about an indication if they
are default settings or have been changed
You can look in the init.ora file for
an indication of manually set parameters. For all parameters, their value and
whether or not the current value is the default value, look in the v$parameter
view.
11. Describe hit ratio as it pertains
to the database buffers. What is the difference between instantaneous and
cumulative hit ratio and which should be used for tuning
The hit ratio is a measure of how many
times the database was able to read a value from the buffers verses how many
times it had to re-read a data value from the disks. A value greater than
80-90% is good, less could indicate problems. If you simply take the ratio of
existing parameters this will be a cumulative value since the database started.
If you do a comparison between pairs of readings based on some arbitrary time
span, this is the instantaneous ratio for that time span. Generally speaking an
instantaneous reading gives more valuable data since it will tell you what your
instance is doing for the time it was generated over.
12. Discuss row chaining, how does it
happen? How can you reduce it? How do you correct it
Row chaining occurs when a VARCHAR2
value is updated and the length of the new value is longer than the old value
and won?t fit in the remaining block space. This results in the row chaining to
another block. It can be reduced by setting the storage parameters on the table
to appropriate values. It can be corrected by export and import of the effected
table.
[edit] Oracle Interview Questions
1. Give one method for transferring a
table from one schema to another:
There are several possible methods,
export-import, CREATE TABLE... AS SELECT, or COPY.
2. What is the purpose of the IMPORT
option IGNORE? What is it?s default setting
The IMPORT IGNORE option tells import
to ignore "already exists" errors. If it is not specified the tables
that already exist will be skipped. If it is specified, the error is ignored
and the tables data will be inserted. The default value is N.
3. You have a rollback segment in a
version 7.2 database that has expanded beyond optimal, how can it be restored
to optimal
Use the ALTER TABLESPACE ..... SHRINK
command.
4. If the DEFAULT and TEMPORARY
tablespace clauses are left out of a CREATE USER command what happens? Is this
bad or good? Why
The user is assigned the SYSTEM
tablespace as a default and temporary tablespace. This is bad because it causes
user objects and temporary segments to be placed into the SYSTEM tablespace
resulting in fragmentation and improper table placement (only data dictionary
objects and the system rollback segment should be in SYSTEM).
5. What are some of the Oracle
provided packages that DBAs should be aware of
Oracle provides a number of packages
in the form of the DBMS_ packages owned by the SYS user. The packages used by
DBAs may include: DBMS_SHARED_POOL, DBMS_UTILITY, DBMS_SQL, DBMS_DDL,
DBMS_SESSION, DBMS_OUTPUT and DBMS_SNAPSHOT. They may also try to answer with
the UTL*.SQL or CAT*.SQL series of SQL procedures. These can be viewed as extra
credit but aren?t part of the answer.
6. What happens if the constraint name
is left out of a constraint clause
The Oracle system will use the default
name of SYS_Cxxxx where xxxx is a system generated number. This is bad since it
makes tracking which table the constraint belongs to or what the constraint
does harder.
7. What happens if a tablespace clause
is left off of a primary key constraint clause
This results in the index that is
automatically generated being placed in then users default tablespace. Since
this will usually be the same tablespace as the table is being created in, this
can cause serious performance problems.
8. What is the proper method for
disabling and re-enabling a primary key constraint
You use the ALTER TABLE command for
both. However, for the enable clause you must specify the USING INDEX and
TABLESPACE clause for primary keys.
9. What happens if a primary key
constraint is disabled and then enabled without fully specifying the index
clause
The index is created in the user?s
default tablespace and all sizing information is lost. Oracle doesn?t store
this information as a part of the constraint definition, but only as part of
the index definition, when the constraint was disabled the index was dropped
and the information is gone.
10. (On UNIX) When should more than
one DB writer process be used? How many should be used
If the UNIX system being used is
capable of asynchronous IO then only one is required, if the system is not
capable of asynchronous IO then up to twice the number of disks used by Oracle
number of DB writers should be specified by use of the db_writers
initialization parameter.
11. You are using hot backup without
being in archivelog mode, can you recover in the event of a failure? Why or why
not
You can?t use hot backup without being
in archivelog mode. So no, you couldn?t recover.
12. What causes the "snapshot too
old" error? How can this be prevented or mitigated
This is caused by large or long
running transactions that have either wrapped onto their own rollback space or
have had another transaction write on part of their rollback space. This can be
prevented or mitigated by breaking the transaction into a set of smaller
transactions or increasing the size of the rollback segments and their extents.
13. How can you tell if a database
object is invalid By checking the status column of the DBA_, ALL_ or
USER_OBJECTS views, depending upon whether you own or only have permission on
the view or are using a DBA account.
13. A user is getting an ORA-00942
error yet you know you have granted them permission on the table, what else
should you check
You need to check that the user has
specified the full name of the object (select empid from scott.emp; instead of
select empid from emp;) or has a synonym that balls to the object (create
synonym emp for scott.emp;)
14. A developer is trying to create a
view and the database won?t let him. He has the "DEVELOPER" role
which has the "CREATE VIEW" system privilege and SELECT grants on the
tables he is using, what is the problem
You need to verify the developer has
direct grants on all tables used in the view. You can?t create a stored object
with grants given through views.
15. If you have an example table, what
is the best way to get sizing data for the production table implementation
The best way is to analyze the table
and then use the data provided in the DBA_TABLES view to get the average row
length and other pertinent data for the calculation. The quick and dirty way is
to look at the number of blocks the table is actually using and ratio the
number of rows in the table to its number of blocks against the number of
expected rows.
16. How can you find out how many
users are currently logged into the database? How can you find their operating
system id
There are several ways. One is to look
at the v$session or v$process views. Another way is to check the current_logins
parameter in the v$sysstat view. Another if you are on UNIX is to do a "ps
-ef|grep oracle|wc -l? command, but this only works against a single instance
installation.
17. A user selects from a sequence and
gets back two values, his select is: SELECT pk_seq.nextval FROM dual;What is
the problem Somehow two values have been inserted into the dual table. This
table is a single row, single column table that should only have one value in
it.
18. How can you determine if an index
needs to be dropped and rebuilt
Run the ANALYZE INDEX command on the
index to validate its structure and then calculate the ratio of
LF_BLK_LEN/LF_BLK_LEN+BR_BLK_LEN and if it isn?t near 1.0 (i.e. greater than
0.7 or so) then the index should be rebuilt. Or if the ratio BR_BLK_LEN/
LF_BLK_LEN+BR_BLK_LEN is nearing 0.3.
[edit] Oracle Interview Questions
1. How can variables be passed to a
SQL routine
By use of the & symbol. For passing
in variables the numbers 1-8 can be used (&1, &2,...,&8) to pass
the values after the command into the SQLPLUS session. To be prompted for a
specific variable, place the ampersanded variable in the code itself:
"select * from dba_tables where owner=&owner_name;" . Use of
double ampersands tells SQLPLUS to resubstitute the value for each subsequent
use of the variable, a single ampersand will cause a reprompt for the value
unless an ACCEPT statement is used to get the value from the user.
2. You want to include a carriage
return/linefeed in your output from a SQL script, how can you do this
The best method is to use the CHR()
function (CHR(10) is a return/linefeed) and the concatenation function
"||". Another method, although it is hard to document and isn?t
always portable is to use the return/linefeed as a part of a quoted string.
3. How can you call a PL/SQL procedure
from SQL
By use of the EXECUTE (short form
EXEC) command.
4. How do you execute a host operating
system command from within SQL
By use of the exclamation ball
"!" (in UNIX and some other OS) or the HOST (HO) command.
5. You want to use SQL to build SQL,
what is this called and give an example
This is called dynamic SQL. An example
would be: set lines 90 pages 0 termout off feedback off verify off spool
drop_all.sql select ?drop user ?||username||? cascade;? from dba_users where
username not in ("SYS?,?SYSTEM?); spool off Essentially you are looking to
see that they know to include a command (in this case DROP USER...CASCADE;) and
that you need to concatenate using the ?||? the values selected from the
database.
6. What SQLPlus command is used to
format output from a select
This is best done with the COLUMN
command.
7. You want to group the following set
of select returns, what can you group on
Max(sum_of_cost), min(sum_of_cost),
count(item_no), item_no The only column that can be grouped on is the
"item_no" column, the rest have aggregate functions associated with
them.
8. What special Oracle feature allows
you to specify how the cost based system treats a SQL statement
The COST based system allows the use
of HINTs to control the optimizer path selection. If they can give some example
hints such as FIRST ROWS, ALL ROWS, USING INDEX, STAR, even better.
9. You want to determine the location
of identical rows in a table before attempting to place a unique index on the
table, how can this be done
Oracle tables always have one
guaranteed unique column, the rowid column. If you use a min/max function
against your rowid and then select against the proposed primary key you can
squeeze out the rowids of the duplicate rows pretty quick. For example: select
rowid from emp e where e.rowid > (select min(x.rowid) from emp x where x.emp_no
= e.emp_no); In the situation where multiple columns make up the proposed key,
they must all be used in the where clause.
10. What is a Cartesian product
A Cartesian product is the result of
an unrestricted join of two or more tables. The result set of a three table
Cartesian product will have x * y * z number of rows where x, y, z correspond
to the number of rows in each table involved in the join.
11. You are joining a local and a
remote table, the network manager complains about the traffic involved, how can
you reduce the network traffic Push the processing of the remote data to the
remote instance by using a view to pre-select the information for the join.
This will result in only the data required for the join being sent across.
11. What is the default ordering of an
ORDER BY clause in a SELECT statement
Ascending
12. What is tkprof and how is it used
The tkprof tool is a tuning tool used
to determine cpu and execution times for SQL statements. You use it by first
setting timed_statistics to true in the initialization file and then turning on
tracing for either the entire database via the sql_trace parameter or for the
session using the ALTER SESSION command. Once the trace file is generated you
run the tkprof tool against the trace file and then look at the output from the
tkprof tool. This can also be used to generate explain plan output.
13. What is explain plan and how is it
used
The EXPLAIN PLAN command is a tool to
tune SQL statements. To use it you must have an explain_table generated in the
user you are running the explain plan for. This is created using the
utlxplan.sql script. Once the explain plan table exists you run the explain
plan command giving as its argument the SQL statement to be explained. The
explain_plan table is then queried to see the execution plan of the statement.
Explain plans can also be run using tkprof.
14. How do you set the number of lines
on a page of output? The width
The SET command in SQLPLUS is used to
control the number of lines generated per page and the width of those lines,
for example SET PAGESIZE 60 LINESIZE 80 will generate reports that are 60 lines
long with a line width of 80 characters. The PAGESIZE and LINESIZE options can
be shortened to PAGES and LINES.
15. How do you prevent output from
coming to the screen
The SET option TERMOUT controls output
to the screen. Setting TERMOUT OFF turns off screen output. This option can be
shortened to TERM.
16. How do you prevent Oracle from
giving you informational messages during and after a SQL statement execution
The SET options FEEDBACK and VERIFY
can be set to OFF.
17. How do you generate file output
from SQL
By use of the SPOOL comm
[edit] Oracle Interview Questions
1. What is a CO-RELATED SUBQUERY
A CO-RELATED SUBQUERY is one that has
a correlation name as table or view designator in the FROM clause of the outer
query and the same correlation name as a qualifier of a search condition in the
WHERE clause of the subquery. eg
SELECT field1 from table1 X
WHERE field2>(select
avg(field2) from table1 Y
where
field1=X.field1);
(The subquery in a correlated subquery
is revaluated for every row of the table or view named in the outer query.)
2. What are various joins used while
writing SUBQUERIES
Self join-Its a join foreign key of a
table references the same table.
Outer Join--Its a join condition used
where One can query all the rows of one of the tables in the join condition
even though they don't satisfy the join condition.
Equi-join--Its a join condition that
retrieves rows from one or more tables in which one or more columns in one
table are equal to one or more columns in the second table.
3. What are various constraints used
in SQL
NULL NOT NULL CHECK DEFAULT
4. What are different Oracle database
objects
TABLES VIEWS INDEXES SYNONYMS
SEQUENCES TABLESPACES etc
5. What is difference between Rename
and Alias
Rename is a permanent name given to a
table or column whereas Alias is a temporary name given to a table or column
which do not exist once the SQL statement is executed.
6. What is a view
A view is stored procedure based on
one or more tables, its a virtual table.
7. What are various privileges that a
user can grant to another user
SELECT CONNECT RESOURCE
8. What is difference between UNIQUE
and PRIMARY KEY constraints
A table can have only one PRIMARY KEY
whereas there can be any number of UNIQUE keys. The columns that compose PK are
automatically define NOT NULL, whereas a column that compose a UNIQUE is not
automatically defined to be mandatory must also specify the column is NOT NULL.
9. Can a primary key contain more than
one columns
Yes
10. How you will avoid duplicating
records in a query
By using DISTINCT
11. What is difference between SQL and
SQL*PLUS
SQL*PLUS is a command line tool where
as SQL and PL/SQL language interface and reporting tool. Its a command line
tool that allows user to type SQL commands to be executed directly against an
Oracle database. SQL is a language used to query the relational
database(DML,DCL,DDL). SQL*PLUS commands are used to format query result, Set
options, Edit SQL commands and PL/SQL.
12. Which datatype is used for storing
graphics and images
LONG RAW data type is used for storing
BLOB's (binary large objects).
13. How will you delete duplicating
rows from a base table
DELETE FROM table_name A WHERE
rowid>(SELECT min(rowid) from table_name B where B.table_no=A.table_no);
CREATE TABLE new_table AS SELECT
DISTINCT * FROM old_table;
DROP old_table RENAME new_table TO
old_table DELETE FROM table_name A WHERE rowid NOT IN (SELECT MAX(ROWID) FROM
table_name GROUP BY column_name)
14. What is difference between SUBSTR
and INSTR
SUBSTR returns a specified portion of
a string eg SUBSTR('BCDEF',4) output BCDE INSTR provides character position in
which a pattern is found in a string.
eg INSTR('ABC-DC-F','-',2) output 7
(2nd occurence of '-')
15. There is a string '120000 12 0
.125' ,how you will find the position of the decimal place
INSTR('120000 12 0 .125','.',1) output
13
16. There is a '%' sign in one field
of a column. What will be the query to find it.
'\' Should be used before '%'.
17. When you use WHERE clause and when
you use HAVING clause
HAVING clause is used when you want to
specify a condition for a group function and it is written after GROUP BY
clause The WHERE clause is used when you want to specify a condition for
columns, single row functions except group functions and it is written before
GROUP BY clause if it is used.
18. Which is more faster - IN or
EXISTS
EXISTS is more faster than IN because
EXISTS returns a Boolean value whereas IN returns a value.
Appropriate answer will be....
Result of the subquery is small Then
"IN" is typicaly more appropriate. and Result of the subquery is
big/large/long Then "EXIST" is more appropriate.
19. What is a OUTER JOIN
Outer Join--Its a join condition used
where you can query all the rows of one of the tables in the join condition
even though they dont satisfy the join condition.
20. How you will avoid your query from
using indexes
SELECT * FROM emp Where emp_no+'
'=12345;
i.e you have to concatenate the column
name with space within codes in the where condition.
SELECT /*+ FULL(a) */ ename, emp_no
from emp where emp_no=1234;
i.e using HINTS
[edit] Oracle Interview Questions
1. What is a pseudo column. Give some
examples
It is a column that is not an actual
column in the table.
eg USER, UID, SYSDATE, ROWNUM, ROWID,
NULL, AND LEVEL.
Suppose customer table is there having
different columns like customer no, payments.What will be the query to select
top three max payments.
For top N queries, see
http://www.orafaq.com/forum/mv/msg/160920/472554/102589/#msg_472554 post
2. What is the purpose of a cluster.
Oracle does not allow a user to
specifically locate tables, since that is a part of the function of the RDBMS.
However, for the purpose of increasing performance, oracle allows a developer
to create a CLUSTER. A CLUSTER provides a means for storing data from different
tables together for faster retrieval than if the table placement were left to
the RDBMS.
3. What is a cursor.
Oracle uses work area to execute SQL
statements and store processing information PL/SQL construct called a cursor
lets you name a work area and access its stored information A cursor is a
mechanism used to fetch more than one row in a Pl/SQl block.
4. Difference between an implicit
& an explicit cursor.
PL/SQL declares a cursor implicitly
for all SQL data manipulation statements, including quries that return only one
row. However,queries that return more than one row you must declare an explicit
cursor or use a cursor FOR loop.
Explicit cursor is a cursor in which
the cursor name is explicitly assigned to a SELECT statement via the
CURSOR...IS statement. An implicit cursor is used for all SQL statements
Declare, Open, Fetch, Close. An explicit cursors are used to process multirow
SELECT statements An implicit cursor is used to process INSERT, UPDATE, DELETE
and single row SELECT. .INTO statements.
5. What are cursor attributes
%ROWCOUNT %NOTFOUND %FOUND %ISOPEN
6. What is a cursor for loop.
Cursor For Loop is a loop where oracle
implicitly declares a loop variable, the loop index that of the same record
type as the cursor's record.
7. Difference between NO DATA FOUND
and %NOTFOUND
NO DATA FOUND is an exception raised
only for the SELECT....INTO statements when the where clause of the querydoes
not match any rows. When the where clause of the explicit cursor does not match
any rows the %NOTFOUND attribute is set to TRUE instead.
8. What a SELECT FOR UPDATE cursor
represent.
SELECT......FROM......FOR......UPDATE[OF
column-reference][NOWAIT] The processing done in a fetch loop modifies the rows
that have been retrieved by the cursor. A convenient way of modifying the rows
is done by a method with two parts: the FOR UPDATE clause in the cursor
declaration, WHERE CURRENT OF CLAUSE in an UPDATE or declaration statement.
9. What 'WHERE CURRENT OF ' clause
does in a cursor.
LOOP
SELECT num_credits
INTO v_numcredits FROM classes
WHERE dept=123 and course=101;
UPDATE students
SET
current_credits=current_credits+v_numcredits
WHERE CURRENT OF
X;
END LOOP
COMMIT;
END;
10. What is use of a cursor variable?
How it is defined.
A cursor variable is associated with
different statements at run time, which can hold different values at run time.
Static cursors can only be associated with one run time query. A cursor
variable is reference type(like a pointer in C). Declaring a cursor variable:
TYPE type_name IS REF CURSOR RETURN return_type type_name is the name of the
reference type,return_type is a record type indicating the types of the select
list that will eventually be returned by the cursor variable.
11. What should be the return type for
a cursor variable.Can we use a scalar data type as return type.
The return type for a cursor must be a
record type.It can be declared explicitly as a user-defined or %ROWTYPE can be
used. eg TYPE t_studentsref IS REF CURSOR RETURN students%ROWTYPE
12. How you open and close a cursor
variable.Why it is required.
OPEN cursor variable FOR
SELECT...Statement CLOSE cursor variable In order to associate a cursor
variable with a particular SELECT statement OPEN syntax is used.In order to free
the resources used for the query CLOSE statement is used.
13. How you were passing cursor
variables in PL/SQL 2.2.
In PL/SQL 2.2 cursor variables cannot
be declared in a package.This is because the storage for a cursor variable has
to be allocated using Pro*C or OCI with version 2.2,the only means of passing a
cursor variable to a PL/SQL block is via bind variable or a procedure
parameter.
14. Can cursor variables be stored in
PL/SQL tables.If yes how.If not why.
No, a cursor variable points a row
which cannot be stored in a two-dimensional PL/SQL table.
15. Difference between procedure and
function.
Functions are named PL/SQL blocks that
return a value and can be called with arguments procedure a named block that
can be called with parameter. A procedure all is a PL/SQL statement by itself,
while a Function call is called as part of an expression.
16. What are different modes of
parameters used in functions and procedures.
IN OUT INOUT
17. What is difference between a
formal and an actual parameter
The variables declared in the
procedure and which are passed, as arguments are called actual, the parameters
in the procedure declaration. Actual parameters contain the values that are
passed to a procedure and receive results. Formal parameters are the
placeholders for the values of actual parameters
18. Can the default values be assigned
to actual parameters.
Yes
19. Can a function take OUT
parameters.If not why.
Yes. A function return a value, but
can also have one or more OUT parameters. it is best practice, however to use a
procedure rather than a function if you have multiple values to return.
20. What is syntax for dropping a
procedure and a function .Are these operations possible.
Drop Procedure procedure_name
Drop Function function_name
21. What are ORACLE PRECOMPILERS.
Using ORACLE PRECOMPILERS ,SQL
statements and PL/SQL blocks can be contained inside 3GL programs written in
C,C++,COBOL,PASCAL, FORTRAN,PL/1 AND ADA. The Precompilers are known as
Pro*C,Pro*Cobol,... This form of PL/SQL is known as embedded pl/sql,the
language in which pl/sql is embedded is known as the host language. The
prcompiler translates the embedded SQL and pl/sql ststements into calls to the
precompiler runtime library.The output must be compiled and linked with this
library to creater an executable.
22. What is OCI. What are its uses.
Oracle Call Interface is a method of
accesing database from a 3GL program. Uses--No precompiler is required,PL/SQL
blocks are executed like other DML statements.
The OCI library provides
-functions to parse SQL
statemets
-bind input variables
-bind output variables
-execute statements
-fetch the results
23. Difference between database
triggers and form triggers.
a) Data base trigger(DBT) fires when a
DML operation is performed on a data base table.Form trigger(FT) Fires when
user presses a key or navigates between fields on the screen b) Can be row
level or statement level No distinction between row level and statement level.
c) Can manipulate data stored in Oracle tables via SQL Can manipulate data in
Oracle tables as well as variables in forms. d) Can be fired from any session
executing the triggering DML statements. Can be fired only from the form that
define the trigger. e) Can cause other database triggers to fire.Can cause
other database triggers to fire,but not other form triggers.
24. What is an UTL_FILE.What are
different procedures and functions associated
with it. UTL_FILE is a package that
adds the ability to read and write to operating system files Procedures
associated with it are FCLOSE, FCLOSE_ALL and 5 procedures to output data to a
file PUT, PUT_LINE, NEW_LINE, PUTF, FFLUSH.PUT,
FFLUSH.PUT_LINE,FFLUSH.NEW_LINE. Functions associated with it are FOPEN, ISOPEN.
25. Can you use a commit statement
within a database trigger.
No
26. What is the maximum buffer size
that can be specified using the DBMS_OUTPUT.ENABLE function?
1,000,000
[edit] Oracle Interview Questions
1. When looking at the estat events
report you see that you are getting busy buffer waits. Is this bad? How can you
find what is causing it
Buffer busy waits could indicate
contention in redo, rollback or data blocks. You need to check the v$waitstat
view to see what areas are causing the problem. The value of the
"count" column tells where the problem is, the "class"
column tells you with what. UNDO is rollback segments, DATA is data base
buffers.
2. If you see contention for library
caches how can you fix it
Increase the size of the shared pool.
3. If you see statistics that deal
with "undo" what are they really talking about
Rollback segments and associated
structures.
4. If a tablespace has a default
pctincrease of zero what will this cause (in relationship to the smon process)
The SMON process won?t automatically
coalesce its free space fragments.
5. If a tablespace shows excessive
fragmentation what are some methods to defragment the tablespace? (7.1,7.2 and
7.3 only)
In Oracle 7.0 to 7.2 The use of the
'alter session set events 'immediate trace name coalesce level ts#';? command
is the easiest way to defragment contiguous free space fragmentation. The ts#
parameter corresponds to the ts# value found in the ts$ SYS table. In version
7.3 the ?alter tablespace coalesce;? is best. If the free space isn?t
contiguous then export, drop and import of the tablespace contents may be the
only way to reclaim non-contiguous free space.
6. How can you tell if a tablespace
has excessive fragmentation
If a select against the dba_free_space
table shows that the count of a tablespaces extents is greater than the count
of its data files, then it is fragmented.
7. You see the following on a status
report: redo log space requests 23 redo log space wait time 0 Is this something
to worry about? What if redo log space wait time is high? How can you fix this
Since the wait time is zero, no. If the wait time was high it might indicate a
need for more or larger redo logs.
8. What can cause a high value for
recursive calls? How can this be fixed
A high value for recursive calls is
cause by improper cursor usage, excessive dynamic space management actions, and
or excessive statement re-parses. You need to determine the cause and correct
it By either relinking applications to hold cursors, use proper space
management techniques (proper storage and sizing) or ensure repeat queries are
placed in packages for proper reuse.
9. If you see a pin hit ratio of less
than 0.8 in the estat library cache report is this a problem? If so, how do you
fix it
This indicate that the shared pool may
be too small. Increase the shared pool size.
10. If you see the value for reloads
is high in the estat library cache report is this a matter for concern
Yes, you should strive for zero
reloads if possible. If you see excessive reloads then increase the size of the
shared pool.
11. You look at the dba_rollback_segs
view and see that there is a large number of shrinks and they are of relatively
small size, is this a problem? How can it be fixed if it is a problem
A large number of small shrinks
indicates a need to increase the size of the rollback segment extents. Ideally
you should have no shrinks or a small number of large shrinks. To fix this just
increase the size of the extents and adjust optimal accordingly.
12. You look at the dba_rollback_segs
view and see that you have a large number of wraps is this a problem
A large number of wraps indicates that
your extent size for your rollback segments are probably too small. Increase
the size of your extents to reduce the number of wraps. You can look at the
average transaction size in the same view to get the information on transaction
size.
[edit] Oracle Interview Questions
1. You have just started a new instance
with a large SGA on a busy existing server. Performance is terrible, what
should you check for
The first thing to check with a large
SGA is that it isn?t being swapped out.
2. What OS user should be used for the
first part of an Oracle installation (on UNIX)
You must use root first.
3. When should the default values for
Oracle initialization parameters be used as is
Never
4. How many control files should you
have? Where should they be located
At least 2 on separate disk spindles.
Be sure they say on separate disks, not just file systems.
5. How many redo logs should you have
and how should they be configured for maximum recoverability
You should have at least three groups
of two redo logs with the two logs each on a separate disk spindle (mirrored by
Oracle). The redo logs should not be on raw devices on UNIX if it can be
avoided.
6. You have a simple application with
no "hot" tables (i.e. uniform IO and access requirements). How many
disks should you have assuming standard layout for SYSTEM, USER, TEMP and
ROLLBACK tablespaces
At least 7, see disk configuration
answer above.
7. Describe third normal form
Something like: In third normal form
all attributes in an entity are related to the primary key and only to the
primary key
8. Is the following statement true or
false:
"All relational databases must be
in third normal form" False. While 3NF is good for logical design most
databases, if they have more than just a few tables, will not perform well
using full 3NF. Usually some entities will be denormalized in the logical to
physical transfer process.
9. What is an ERD
An ERD is an
Entity-Relationship-Diagram. It is used to show the entities and relationships
for a database logical model.
10. Why are recursive relationships
bad? How do you resolve them
A recursive relationship (one where a
table relates to itself) is bad when it is a hard relationship (i.e. neither
side is a "may" both are "must") as this can result in it
not being possible to put in a top or perhaps a bottom of the table (for
example in the EMPLOYEE table you couldn?t put in the PRESIDENT of the company
because he has no boss, or the junior janitor because he has no subordinates).
These type of relationships are usually resolved by adding a small intersection
entity.
11. What does a hard one-to-one
relationship mean (one where the relationship on both ends is "must")
Expected answer: This means the two
entities should probably be made into one entity.
12. How should a many-to-many
relationship be handled
By adding an intersection entity table
13. What is an artificial (derived)
primary key? When should an artificial (or derived) primary key be used
A derived key comes from a sequence.
Usually it is used when a concatenated key becomes too cumbersome to use as a
foreign key.
[edit] Oracle Interview Questions
1. When should you consider
denormalization
Whenever performance analysis
indicates it would be beneficial to do so without compromising data integrity.
2. How can you determine if an Oracle
instance is up from the operating system level
There are several base Oracle
processes that will be running on multi-user operating systems, these will be
smon, pmon, dbwr and lgwr. Any answer that has them using their operating
system process showing feature to check for these is acceptable. For example,
on UNIX a ps -ef|grep dbwr will show what instances are up.
3. Users from the PC clients are
getting messages indicating : ORA-06114: (Cnct err, can't get err txt. See
Servr Msgs & Codes Manual)
What could the problem be The instance
name is probably incorrect in their connection string.
4. Users from the PC clients are
getting the following error stack: ERROR: ORA-01034: ORACLE not available
ORA-07318: smsget: open error when opening sgadef.dbf file. HP-UX Error: 2: No
such file or directory
What is the probable cause The Oracle
instance is shutdown that they are trying to access, restart the instance.
5. How can you determine if the SQLNET
process is running for SQLNET V1? How about V2
For SQLNET V1 check for the existence
of the orasrv process. You can use the command "tcpctl status" to get
a full status of the V1 TCPIP server, other protocols have similar command
formats. For SQLNET V2 check for the presence of the LISTENER process(s) or you
can issue the command "lsnrctl status".
6. What file will give you Oracle
instance status information? Where is it located
The alert.ora log. It is located in
the directory specified by the background_dump_dest parameter in the
v$parameter table.
7. Users aren?t being allowed on the
system. The following message is received: ORA-00257 archiver is stuck. Connect
internal only, until freed What is the problem The archive destination is
probably full, backup the archive logs and remove them and the archiver will
re-start.
8. Where would you look to find out if
a redo log was corrupted assuming you are using Oracle mirrored redo logs
There is no message that comes to the
SQLDBA or SRVMGR programs during startup in this situation, you must check the
alert.log file for this information.
9. You attempt to add a datafile and
get: ORA-01118: cannot add anymore datafiles: limit of 40 exceeded What is the
problem and how can you fix it When the database was created the db_files
parameter in the initialization file was set to 40. You can shutdown and reset
this to a higher value, up to the value of MAX_DATAFILES as specified at
database creation. If the MAX_DATAFILES is set to low, you will have to rebuild
the control file to increase it before proceeding.
10. You look at your fragmentation
report and see that smon hasn?t coalesced any of you tablespaces, even though
you know several have large chunks of contiguous free extents. What is the
problem
Check the dba_tablespaces view for the
value of pct_increase for the tablespaces. If pct_increase is zero, smon will
not coalesce their free space.
11. Your users get the following
error: ORA-00055 maximum number of DML locks exceeded What is the problem and
how do you fix it The number of DML Locks is set by the initialization
parameter DML_LOCKS. If this value is set to low (which it is by default) you
will get this error. Increase the value of DML_LOCKS. If you are sure that this
is just a temporary problem, you can have them wait and then try again later
and the error should clear.
12. You get a call from you backup DBA
while you are on vacation. He has corrupted all of the control files while
playing with the ALTER DATABASE BACKUP CONTROLFILE command. What do you do
As long as all datafiles are safe and
he was successful with the BACKUP controlfile command you can do the following:
CONNECT INTERNAL STARTUP MOUNT (Take any read-only tablespaces offline before
next step ALTER DATABASE DATAFILE .... OFFLINE;) RECOVER DATABASE USING BACKUP
CONTROLFILE ALTER DATABASE OPEN RESETLOGS; (bring read-only tablespaces back
online) Shutdown and backup the system, then restart If they have a recent
output file from the ALTER DATABASE BACKUP CONTROL FILE TO TRACE; command, they
can use that to recover as well. If no backup of the control file is available
then the following will be required: CONNECT INTERNAL STARTUP NOMOUNT CREATE
CONTROL FILE .....; However, they will need to know all of the datafiles,
logfiles, and settings for MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY,
MAXDATAFILES for the database to use the command.
[edit] Oracle Interview Questions
1. How would you determine the time
zone under which a database was operating? 2. Explain the use of setting
GLOBAL_NAMES equal to TRUE. 3. What command would you use to encrypt a PL/SQL
application? 4. Explain the difference between a FUNCTION, PROCEDURE and
PACKAGE. 5. Explain the use of table functions. 6. Name three advisory
statistics you can collect. 7. Where in the Oracle directory tree structure are
audit traces placed? 8. Explain materialized views and how they are used. 9.
When a user process fails, what background process cleans up after it? 10. What
background process refreshes materialized views? 11. How would you determine
what sessions are connected and what resources they are waiting for? 12.
Describe what redo logs are. 13. How would you force a log switch? 14. Give two
methods you could use to determine what DDL changes have been made. 15. What
does coalescing a tablespace do? 16. What is the difference between a TEMPORARY
tablespace and a PERMANENT tablespace? 17. Name a tablespace automatically
created when you create a database. 18. When creating a user, what permissions
must you grant to allow them to connect to the database? 19. How do you add a
data file to a tablespace? 20. How do you resize a data file? 21. What view
would you use to look at the size of a data file? 22. What view would you use
to determine free space in a tablespace? 23. How would you determine who has added
a row to a table? 24. How can you rebuild an index? 25. Explain what
partitioning is and what its benefit is. 26. You have just compiled a PL/SQL
package but got errors, how would you view the errors? 27. How can you gather
statistics on a table? 28. How can you enable a trace for a session? 29. What
is the difference between the SQL*Loader and IMPORT utilities? 30. Name two
files used for network connection to a database.
[edit] Oracle Interview Questions
1. In a system with an average of 40
concurrent users you get the following from a query on rollback extents:
ROLLBACK CUR EXTENTS
--------------------------
R01 11 R02 8 R03 12 R04 9 SYSTEM 4 2.
You have room for each to grow by 20 more extents each. Is there a problem?
Should you take any action
No there is not a problem. You have 40
extents showing and an average of 40 concurrent users. Since there is plenty of
room to grow no action is needed.
3. You see multiple extents in the
temporary tablespace. Is this a problem
As long as they are all the same size
this isn?t a problem. In fact, it can even improve performance since Oracle
won?t have to create a new extent when a user needs one.
4. Define OFA.
OFA stands for Optimal Flexible
Architecture. It is a method of placing directories and files in an Oracle
system so that you get the maximum flexibility for future tuning and file
placement.
5. How do you set up your tablespace
on installation
The answer here should show an
understanding of separation of redo and rollback, data and indexes and
isolation os SYSTEM tables from other tables. An example would be to specify
that at least 7 disks should be used for an Oracle installation so that you can
place SYSTEM tablespace on one, redo logs on two (mirrored redo logs) the
TEMPORARY tablespace on another, ROLLBACK tablespace on another and still have
two for DATA and INDEXES. They should indicate how they will handle archive
logs and exports as well. As long as they have a logical plan for combining or
further separation more or less disks can be specified.
6. What should be done prior to
installing Oracle (for the OS and the disks)
adjust kernel parameters or OS tuning
parameters in accordance with installation guide. Be sure enough contiguous
disk space is available.
7. You have installed Oracle and you
are now setting up the actual instance. You have been waiting an hour for the
initialization script to finish, what should you check first to determine if
there is a problem
Check to make sure that the archiver
isn?t stuck. If archive logging is turned on during install a large number of
logs will be created. This can fill up your archive log destination causing
Oracle to stop to wait for more space.
8. When configuring SQLNET on the
server what files must be set up
INITIALIZATION file, TNSNAMES.ORA
file, SQLNET.ORA file
9. When configuring SQLNET on the
client what files need to be set up
SQLNET.ORA, TNSNAMES.ORA
10. What must be installed with ODBC
on the client in order for it to work with Oracle
SQLNET and PROTOCOL (for example:
TCPIP adapter) layers of the transport programs.
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