50 TOP PLSQL Triggers Interview Questions and Answers

PL/SQL interview questions and answers - most frequently asked PL/SQL interview questions and answers.

1. 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.

2. 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.

7. 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.

9. 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.

10. 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.

11. 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.

12. 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

13. 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>

14. 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.

15. 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

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

32. 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.

33. 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.

34. 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.

35. 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.

36. Which of the following events cannot be customized with triggers?
a. INSERT INTO statement
b. CREATE TABLE statement in a schema
c. DELETE CASCADE CONSTRAINTS
d. SET PAUSE ON
Option D cannot be customized; whereas, all the other options can be customized using triggers.

37. 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.

38. 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.

39. Can cursors be part of a trigger body?
Yes, cursors can be a part of code in trigger.

42. 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.

43. 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.

44. 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.

46. 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.

47. 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.

48. 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.

49. 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;

50. 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.