Oracle Database Interview Questions - 1
Differentiate between TRUNCATE and DELETE
What is the maximum buffer size that can be specified using the DBMS_OUTPUT.ENABLE function?
Can you use a commit statement within a database trigger?
What is an UTL_FILE.What are different procedures and functions associated with it?
Difference between database triggers and form triggers?
What is OCI. What are its uses?
What are ORACLE PRECOMPILERS?
What is syntax for dropping a procedure and a function? Are these operations possible?
Can a function take OUT parameters. If not why?
Can the default values be assigned to actual parameters?
What is difference between a formal and an actual parameter?
What are different modes of parameters used in functions and procedures?
Difference between procedure and function.
Can cursor variables be stored in PL/SQL tables.If yes how. If not why?
How do you pass cursor variables in PL/SQL?
How do you open and close a cursor variable.Why it is required?
What should be the return type for a cursor variable.Can we use a scalar data type as return type?
What is use of a cursor variable? How it is defined?
What WHERE CURRENT OF clause does in a cursor?
Difference between NO DATA FOUND and %NOTFOUND
What is a cursor for loop?
What are cursor attributes?
Difference between an implicit & an explicit cursor.
What is a cursor?
What is the purpose of a cluster?
How do you find the numbert of rows in a Table ?
Display the number value in Words?
What is a pseudo column. Give some examples?
How you will avoid your query from using indexes?
What is a OUTER JOIN?
Which is more faster - IN or EXISTS?
When do you use WHERE clause and when do you use HAVING clause?
There is a % sign in one field of a column. What will be the query to find it?
What is difference between SUBSTR and INSTR?
Which datatype is used for storing graphics and images?
What is difference between SQL and SQL*PLUS?
What is difference between UNIQUE and PRIMARY KEY constraints?
What is difference between Rename and Alias?
What are various joins used while writing SUBQUERIES?
What is the maximum buffer size that can be specified using the DBMS_OUTPUT.ENABLE function?
Can you use a commit statement within a database trigger?
What is an UTL_FILE.What are different procedures and functions associated with it?
Difference between database triggers and form triggers?
What is OCI. What are its uses?
What are ORACLE PRECOMPILERS?
What is syntax for dropping a procedure and a function? Are these operations possible?
Can a function take OUT parameters. If not why?
Can the default values be assigned to actual parameters?
What is difference between a formal and an actual parameter?
What are different modes of parameters used in functions and procedures?
Difference between procedure and function.
Can cursor variables be stored in PL/SQL tables.If yes how. If not why?
How do you pass cursor variables in PL/SQL?
How do you open and close a cursor variable.Why it is required?
What should be the return type for a cursor variable.Can we use a scalar data type as return type?
What is use of a cursor variable? How it is defined?
What WHERE CURRENT OF clause does in a cursor?
Difference between NO DATA FOUND and %NOTFOUND
What is a cursor for loop?
What are cursor attributes?
Difference between an implicit & an explicit cursor.
What is a cursor?
What is the purpose of a cluster?
How do you find the numbert of rows in a Table ?
Display the number value in Words?
What is a pseudo column. Give some examples?
How you will avoid your query from using indexes?
What is a OUTER JOIN?
Which is more faster - IN or EXISTS?
When do you use WHERE clause and when do you use HAVING clause?
There is a % sign in one field of a column. What will be the query to find it?
What is difference between SUBSTR and INSTR?
Which datatype is used for storing graphics and images?
What is difference between SQL and SQL*PLUS?
What is difference between UNIQUE and PRIMARY KEY constraints?
What is difference between Rename and Alias?
What are various joins used while writing SUBQUERIES?
Part - 2:
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 where 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 a now rollback. The delete command, on the other hand, is a DML operation, which will produce a rollback 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 two examples of referential integrity constraints.
A primary key and a foreign key.
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.
Well, we have gone through the first 25 questions as I would answer them during an interview. Please feel free to add your personal experiences to the answers as it will always improve the process and add your particular touch. As always remember these are "core" DBA questions and not necessarily related to the Oracle options that you may encounter in some interviews. Take a close look at the requirements for any job and try to come up with questions that the interviewer may ask. Next time we will tackle the rest of the questions. Until then, good luck with the process.
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 where 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 a now rollback. The delete command, on the other hand, is a DML operation, which will produce a rollback 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 two examples of referential integrity constraints.
A primary key and a foreign key.
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.
Well, we have gone through the first 25 questions as I would answer them during an interview. Please feel free to add your personal experiences to the answers as it will always improve the process and add your particular touch. As always remember these are "core" DBA questions and not necessarily related to the Oracle options that you may encounter in some interviews. Take a close look at the requirements for any job and try to come up with questions that the interviewer may ask. Next time we will tackle the rest of the questions. Until then, good luck with the process.
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