PL/SQL Interview Questions & Answers

Best PL/SQL Interview Questions and Answers

Dear Readers, Welcome to PL/SQL Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of PL/SQL. These PL/SQL Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT companies.

1. Explain the structure of PL/SQL in brief.

PL/SQL is a procedural language which has interactive SQL, as well as procedural programming language constructs like conditional branching and iteration.

2. Differentiate between % ROWTYPE and TYPE RECORD.

% ROWTYPE is used when a query returns an entire row of a table or view.

TYPE RECORD, on the other hand, is used when a query returns column of different tables or views.

Eg.  TYPE r_emp is RECORD (sno smp.smpno%type,sname smp sname %type)

e_rec smp ROWTYPE

Cursor c1 is select smpno,dept from smp;

e_rec c1 %ROWTYPE

 

3. Explain uses of cursor.

Cursor is a named private area in SQL from which information can be accessed. They are required to process each row individually for queries which return multiple rows.

 

4. Show code of a cursor for loop.

Cursor declares %ROWTYPE as loop index implicitly. It then opens a cursor, gets rows of values from the active set in fields of the record and shuts when all records are processed.

Eg.  FOR smp_rec IN C1 LOOP

totalsal=totalsal+smp_recsal;

ENDLOOP;

 

5. Explain the uses of database trigger.

A PL/SQL program unit associated with a particular database table is called a database trigger. It is used for :

1)Audit data modifications.

2)Log events transparently.

3)Enforce complex business rules.

4)Maintain replica tables

5)Derive column values

6)Implement Complex security authorizations

 

6. What are the two types of exceptions.

Error handling part of PL/SQL block is called Exception. They have two types : user_defined and predefined.

 

7. Show some predefined exceptions.

DUP_VAL_ON_INDEX

ZERO_DIVIDE

NO_DATA_FOUND

TOO_MANY_ROWS

CURSOR_ALREADY_OPEN

INVALID_NUMBER

INVALID_CURSOR

PROGRAM_ERROR

TIMEOUT _ON_RESOURCE

STORAGE_ERROR

LOGON_DENIED

VALUE_ERROR

etc.

 

8. Explain Raise_application_error.

It is a procedure of package DBMS_STANDARD that allows issuing of user_defined error messages from database trigger or stored sub-program.

 

9.Show how functions and procedures are called in a PL/SQL block.

Function is called as a part of an expression.

total:=calculate_sal(‘b644’)

Procedure as a statement in PL/SQL.

calculate_bonus(‘b644’);

 

10. Explain two virtual tables available at the time of database trigger execution.

Table columns are referred as THEN.column_name and NOW.column_name.

For INSERT related triggers, NOW.column_name values are available only.

For DELETE related triggers, THEN.column_name values are available only.

For UPDATE related triggers, both Table columns are available.

 

11. What are the rules to be applied to NULLs whilst doing comparisons?

1) NULL is never TRUE or FALSE

2) NULL cannot be equal or unequal to other values

3) If a value in an expression is NULL, then the expression itself evaluates to NULL except for concatenation operator (||)

 

12. How is a process of PL/SQL compiled?

Compilation process includes syntax check, bind and p-code generation processes.

Syntax checking checks the PL/SQL codes for compilation errors. When all errors are corrected, a storage address is assigned to the variables that hold data. It is called Binding. P-code is a list of instructions for the PL/SQL engine. P-code is stored in the database for named blocks and is used the next time it is executed.

 

13. Differentiate between Syntax and runtime errors.

A syntax error can be easily detected by a PL/SQL compiler. For eg, incorrect spelling.

A runtime error is handled with the help of exception-handling section in an PL/SQL block. For eg, SELECT INTO statement, which does not return any rows.

 

14. Explain Commit, Rollback and Savepoint.

For a COMMIT statement, the following is true:

Other users can see the data changes made by the transaction.

The locks acquired by the transaction are released.

The work done by the transaction becomes permanent.

A ROLLBACK statement gets issued when the transaction ends, and the following is true.

The work done in a transition is undone as if it was never issued.

All locks acquired by transaction are released.

It undoes all the work done by the user in a transaction. With SAVEPOINT, only part of transaction can be undone.

 

15. Define Implicit and Explicit Cursors.

A cursor is implicit by default. The user cannot control or process the information in this cursor.

If a query returns multiple rows of data, the program defines an explicit cursor. This allows the application to process each row sequentially as the cursor returns it.

 

16. Explain mutating table error.

It occurs when a trigger tries to update a row that it is currently using. It is fixed by using views or temporary tables, so database selects one and updates the other.

 

17. When is a declare statement required?

DECLARE statement is used by PL/SQL anonymous blocks such as with stand alone, non-stored procedures. If it is used, it must come first in a stand alone file.

 

18. How many triggers can be applied to a table?

A maximum of 12 triggers can be applied to one table.

 

19. What is the importance of SQLCODE and SQLERRM?

SQLCODE returns the value of the number of error for the last encountered error whereas SQLERRM returns the message for the last error.

 

20. If a cursor is open, how can we find in a PL/SQL Block?

the %ISOPEN cursor status variable can be used.

 

21. Show the two PL/SQL cursor exceptions.

Cursor_Already_Open

Invaid_cursor

 

22. What operators deal with NULL?

NVL converts NULL to another specified value.

var:=NVL(var2,’Hi’);

IS NULL and IS NOT NULL can be used to check specifically to see whether the value of a variable is NULL or not.

 

23. Does SQL*Plus also have a PL/SQL Engine?

No, SQL*Plus does not have a PL/SQL Engine embedded in it. Thus, all PL/SQL code is sent directly to database engine. It is much more efficient as each statement is not individually stripped off.

 

24. What packages are available to PL/SQL developers?

DBMS_ series of packages, such as, DBMS_PIPE, DBMS_DDL, DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_SQL, DBMS_TRANSACTION, UTL_FILE.

 

25. Explain 3 basic parts of a trigger.

A triggering statement or event.

A restriction

An action

 

26. What are character functions?

INITCAP, UPPER, SUBSTR, LOWER and LENGTH are all character functions. Group functions give results based on groups of rows, as opposed to individual rows. They are MAX, MIN, AVG, COUNT and SUM.

 

27. Explain TTITLE and BTITLE.

TTITLE and BTITLE commands that control report headers and footers.

 

28. Show the cursor attributes of PL/SQL.

%ISOPEN : Checks if the cursor is open or not

%ROWCOUNT : The number of rows that are updated, deleted or fetched.

%FOUND : Checks if the cursor has fetched any row. It is true if rows are fetched

%NOT FOUND : Checks if the cursor has fetched any row. It is True if rows are not fetched.

 

29. What is an Intersect?

Intersect is the product of two tables and it lists only matching rows.

 

30. What are sequences?

Sequences are used to generate sequence numbers without an overhead of locking. Its drawback is that the sequence number is lost if the transaction is rolled back.

 

31. How would you reference column values BEFORE and AFTER you have inserted and deleted triggers?

Using the keyword “new.column name”, the triggers can reference column values by new collection. By using the keyword “old.column name”, they can reference column vaues by old collection.

 

32. What are the uses of SYSDATE and USER keywords?

SYSDATE refers to the current server system date. It is a pseudo column. USER is also a pseudo column but refers to current user logged onto the session. They are used to monitor changes happening in the table.

 

33. How does ROWID help in running a query faster?

ROWID is the logical address of a row, it is not a physical column. It composes of data block number, file number and row number in the data block. Thus, I/O time gets minimized retrieving the row, and results in a faster query.

 

34. What are database links used for?

Database links are created in order to form communication between various databases, or different environments like test, development and production. The database links are read-only to access other information as well.

 

35. What does fetching a cursor do?

Fetching a cursor reads Result Set row by row.

 

36. What does closing a cursor do?

Closing a cursor clears the private SQL area as well as de-allocates memory

 

37. Explain the uses of Control File.

It is a binary file. It records the structure of the database. It includes locations of several log files, names and timestamps. They can be stored in different locations to help in retrieval of information if one file gets corrupted.

 

38.  Explain Consistency

Consistency shows that data will not be reflected to other users until the data is commit, so that consistency is maintained.

 

39. Differ between Anonymous blocks and sub-programs.

Anonymous blocks are unnamed blocks that are not stored anywhere whilst sub-programs are compiled and stored in database. They are compiled at runtime.

 

40. Differ between DECODE and CASE.

DECODE and CASE statements are very similar, but CASE is extended version of DECODE. DECODE does not allow Decision making statements in its place.

select decode(totalsal=12000,’high’,10000,’medium’) as decode_tesr from smp where smpno in (10,12,14,16);

This statement returns an error.

CASE is directly used in PL/SQL, but DECODE is used in PL/SQL through SQL only.

 

41. Explain autonomous transaction.

An autonomous transaction is an independent transaction of the main or parent transaction. It is not nested if it is started by another transaction.

There are several situations to use autonomous transactions like event logging and auditing.

 

42. Differentiate between SGA and PGA.

SGA stands for System Global Area whereas PGA stands for Program or Process Global Area. PGA is only allocated 10% RAM size, but SGA is given 40% RAM size.

 

43. What is the location of Pre_defined_functions.

They are stored in the standard package called “Functions, Procedures and Packages”

 

44. Explain polymorphism in PL/SQL.

Polymorphism is a feature of OOP. It is the ability to create a variable, an object or function with multiple forms. PL/SQL supports Polymorphism in the form of program unit overloading inside a member function or package..Unambiguous logic must be avoided whilst overloading is being done.

 

45. What are the uses of MERGE?

MERGE is used to combine multiple DML statements into one.

Syntax : merge into tablename

using(query)

on(join condition)

when not matched then

[insert/update/delete] command

when matched then

[insert/update/delete] command

 

46. Can 2 queries be executed simultaneously in a Distributed Database System?

Yes, they can be executed simultaneously. One query is always independent of the second query in a distributed database system based on the 2 phase commit.

 

47. Explain Raise_application_error.

It is a procedure of the package DBMS_STANDARD that allow issuing a user_defined error messages from the database trigger or stored sub-program.

 

48.  What is out parameter used for eventhough return statement can also be used in pl/sql?

Out parameters allows more than one value in the calling program. Out parameter is not recommended in functions. Procedures can be used instead of functions if multiple values are required. Thus, these procedures are used to execute Out parameters.

 

49. How would you convert date into Julian date format?

We can use the J format string :

SQL > select to_char(to_date(‘29-Mar-2013’,’dd-mon-yyyy’),’J’) as julian from dual;

JULIAN

 

50. Explain SPOOL

Spool command can print the output of sql statements in a file.

spool/tmp/sql_outtxt

select smp_name, smp_id from smp where dept=’accounts’;

spool off;

 

51. What are the system privileges that are required by a schema owner (user) to create a trigger on a table?

A user must be able to alter a table to create a trigger on the table. The user must own the table and either have the ALTER TABLE privilege on that table or have the ALTER ANY TABLE system privilege. In addition, the user must have the CREATE TRIGGER system privilege. User should have the CREATE ANY

TRIGGER system privilege to be able to create triggers in any other user account or schema.

A database-level event trigger can be created if the user has the ADMINISTER DATABASE TRIGGER system privilege.

 

52. What are the different types of triggers?

There are following two types of triggers:

 Database triggers are executed implicitly whenever a Data Manipulation Language (DML) statement is carried out on a  database table or a Data Definition Language (DDL) statement, such as CREATE OR ALTER, is performed on the database.  

They may also be executed when a user or database event occurs, such as a user logs on or a database is shutdown.

Application triggers are executed implicitly whenever a DML event takes place within an application, such as  WHEN_NEW_FORM_INSTANCE in the Oracle Forms application.

 

53. How can triggers be used for the table auditing?

 Triggers can be used to track values for data operations on tables. This is done using the old and new qualifiers within the trigger code. These two clauses help keep track of the data that is being inserted, updated, or deleted in the table; and therefore, facilitate in application auditing of DML statements. The audit trail can be written to a user-defined table and audit records can be generated for both row-level and statement-level triggers.

 

54. What are INSTEAD OF triggers?

The INSTEAD OF triggers are used in association with views. The standard table-based triggers cannot be used by views. 

These triggers inform the database of what actions are to be performed instead of the actions that invoked the trigger. 

Therefore, the INSTEAD OF triggers can be used to update the underlying tables, which are part of the views. 

They can be used on both relational views and object views. The INSTEAD OF triggers can only be defined as row-level triggers and not as statement-level triggers.

 

55. What is the difference between database trigger and stored procedure?

 The main difference between database trigger and stored procedure is that the trigger is invoked implicitly and stored procedure is invoked explicitly.

Transaction Control statements, such as COMMIT, ROLLBACK, and SAVEPOINT, are not allowed within the body of a trigger whereas, these statements can be included in a stored procedure.

 

56. How can the performance of a trigger be improved?

 The performance of a trigger can be improved by using column names along with the UPDATE clause in the trigger. This will make the trigger fire when that particular column is updated and therefore, prevents unnecessary action of trigger when other columns are being updated.

 

57. What are the events on which a database trigger can be based?

 Database triggers are based on system events and can be defined at database or schema level. The various events on

which a database trigger can be based are given as follows:

Data definition statement on a database or schema object

Logging off or on of a specific user

Database shutdown or startup

On any specific error that occurs

 

58. What is a CALL statement? Explain with an example.

A CALL statement within a trigger enables you to call a stored procedure within the trigger rather than writing the Procedural

Language/Structured Query Language (PL/SQL) code in it, The procedure may be in PL/SQL, C, or Java language. Following is

an example of the CALL statement:

CREAT OR REPLASE TRIGGER<trigger_name>

BEFORE UPDATE OF <column_name> ON <table_name>

FOR EACH ROW

WHEN <condition_clause>

CALL <procedure_name>

 

59. What is a mutating table?

A mutating table is a table, which is in the state of transition. In other words, it is a table, which is being updated at the time of triggering action. If the trigger code queries this table, then a mutating table error occurs, which causes the trigger to view the inconsistent data.

 

60. Which data dictionary views have the information on the triggers that are available in the database?

The data dictionary views that have information on database triggers are given as follows:

USER_OBJECTS —Contain the name and status of a trigger as well as the date and time of trigger creation

USER_ERRORS—Contain the compilation error of a trigger

USER_ TRIGGERS— Contain the source code of a trigger

USER_ TRIGGER_COLS—Contain the information on columns used in triggers

 

62. What are schema-level triggers?

Schema-level triggers are created on schema-level operations, such as create table, alter table, drop table, rename, truncate, and revoke. These triggers prevent DDL statements, provide security, and monitor the DDL operations.

 

63. What is a database event trigger?

 Trigger that is executed when a database event, such as startup, shutdown, or error, occurs is called a database event trigger. It can be used to reference the attributes of the event and perform system maintenance functions immediately after the database startup.

 

64. In what condition is it good to disable a trigger?

 It is good to disable triggers during data load operations. This improves the performance of the data loading activities. The data modification and manipulation that the trigger would have performed has to be done manually after the data loading.

 

65. Which column of the USERJTRIGGERS data dictionary view displays the database event that will fire the trigger?

 The Description column of the USERJTRIGGERS view combines information from many columns to display the trigger header, which includes the database event.

 

66. What is the meaning of disabling a trigger?

When a trigger is disabled, it does not mean that it is deleted.

The code of the trigger is still stored in the data dictionary but the trigger will not have any effect on the table.

 

67. Can triggers stop a DML statement from executing on a table?

Yes, triggers have the capability of stopping any DML statement from execution on a table. Any logical business rule can be implemented using PL/SQL to block modification on table data.

 

68. Can a view be mutating? If yes, then how?

 No, a view cannot be mutating like a table. If an UPDATE statement fires an INSTEAD OF trigger on a view, the view is not considered to be mutating. If the UPDATE statement had been executed on a table, the table would have been considered as mutating.

 

69. Can a COMMIT statement be executed as part of a trigger?

No, A COMMIT statement cannot be executed as a part of a trigger because it is a Transaction Control statement, which cannot be executed within a trigger body. Triggers fire within transactions and cannot include any Transaction Control statement within its code.

 

70. What is the difference between ALTER TRIGGER and DROP TRIGGER statements?

 An ALTER TRIGGER statement is used to recompile, disable, or enable a trigger; whereas, the DROP TRIGGER statement is used to remove the trigger from the database.

 

71. Do  triggers  have  restrictions  on the  usage  of large datatypes, such as LONG and LONG RAW?

 Triggers have restrictions on the usage of large datatypes as they cannot declare or reference the LONG and LONG RAW datatypes and cannot use them even if they form part of the object with which the trigger is associated. Similarly, triggers cannot modify the CLOB and BLOB objects as well however, they can reference them for read-only access.

 

72. Are  DDL triggers fired for DDL statements within a PL/SQL code executed using the DBMS.SQL package?

No, DDL triggers are not executed for DDL statements, which are executed within the PL/SQL code using the DBMS_SQL package.

 

73. Does a USER_OBJECTS view have an entry for a trigger?

 Yes, the USER_OBJECTS view has one row entry for each trigger in the schema.

 

74. How can you view the errors encountered in a trigger?

 The USERJERRORS view can be used to show all the parsing errors that occur in a trigger during the compilation until they are resolved.

 

75. Does USERJTRIGGERS have entry for triggers with compilation errors?

 Yes, USER_TRIGGERS have entries for all triggers that are created in the schema with or without errors.

 

76. Is it possible to pass parameters to triggers?

 No, it is not possible to pass parameters to triggers. However, triggers fired by INSERT and UPDATE statements can reference new data by using the mew prefix. In addition, the triggers fired in response to UPDATE and DELETE statements can reference old, modified, or deleted data using the :old prefix.

 

77. Can a SELECT statement fire a trigger?

 No, a SELECT statement cannot fire a trigger. DML statements, such as INSERT, UPDATE, and DELETE, can cause triggers to fire.

 

78. Can cursors be part of a trigger body?

 Yes, cursors can be a part of code in trigger.

 

79. Is it possible to create STARTUP or SHUTDOWN trigger for ON-SCHEMA?

 No, It is not possible to create STARTUP or SHUTDOWN triggers for ON-SCHEMA.

 

80. What does the BASE_OBJECT_TYPE column shows in the USER.TRIGGERS data dictionary view?

 The BASE_OBJECT_TYPE column identifies the type of database object with which the trigger is associated. It shows whether the object of database is a TABLE, VIEW, SCHEMA, or DATABASE.

 

81. Is it possible to create the following trigger: BEFORE OR AFTER UPDATE trigger FOR EACH ROW?

 No, it is an invalid trigger as both BEFORE and AFTER cannot be used in the same trigger. A trigger can be either BEFORE TRIGGER or AFTER TRIGGER.

 

82. Can INSTEAD OF triggers be used to fire once for each statement on a view?

 No, INSTEAD OF triggers cannot be used for each statement however, It can only be used for each row on a view.

 

83. Is it possible to include an INSERT statement on the same table to which the trigger is assigned?

 If an INSERT statement is used on the same table to which the trigger is associated, then this will give rise to a mutating table, as it is not possible to change the same table that is already in the process of being changed.

 

84. What are conditional predicates?

 Triggers use conditional predicates, such as INSERTING, UPDATING, and DELETING, to determine which particular event will cause the trigger to fire. All the three predicates have Boolean values and are useful in triggers, such as AFTER INSERT or UPDATE.

 

85. Write the ALTER statement to enable all the triggers on the T.STUDENTS table.

The ALTER statement is given as follows:

ALTER TABLE T_STUDENTS ENABLE ALL TRIGGERS;

 

86. Which column in the USER.TRIGGERS data dictionary view shows that the trigger is a PL/SQL trigger?

The ACTION_TYPE column of the USER_TRIGGERS data dictionary view shows that the trigger is a PL/SQL trigger.