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.
PL/SQL is a procedural language which has interactive SQL, as well as procedural programming language constructs like conditional branching and iteration.
% 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
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.
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
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
Error handling part of PL/SQL block is called Exception. They have two types : user_defined and predefined.
It is a procedure of package DBMS_STANDARD that allows issuing of user_defined error messages from database trigger or stored sub-program.
Function is called as a part of an expression.
Procedure as a statement in PL/SQL.
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.
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 (||)
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.
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.
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.
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.
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.
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.
A maximum of 12 triggers can be applied to one table.
SQLCODE returns the value of the number of error for the last encountered error whereas SQLERRM returns the message for the last error.
the %ISOPEN cursor status variable can be used.
NVL converts NULL to another specified value.
IS NULL and IS NOT NULL can be used to check specifically to see whether the value of a variable is NULL or not.
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.
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.
A triggering statement or event.
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.
TTITLE and BTITLE commands that control report headers and footers.
%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.
Intersect is the product of two tables and it lists only matching rows.
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.
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.
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.
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.
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.
Fetching a cursor reads Result Set row by row.
Closing a cursor clears the private SQL area as well as de-allocates memory
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.
Consistency shows that data will not be reflected to other users until the data is commit, so that consistency is maintained.
Anonymous blocks are unnamed blocks that are not stored anywhere whilst sub-programs are compiled and stored in database. They are compiled at runtime.
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.
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.
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.
They are stored in the standard package called “Functions, Procedures and Packages”
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.
MERGE is used to combine multiple DML statements into one.
Syntax : merge into tablename
when not matched then
when matched then
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.
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.
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.
We can use the J format string :
SQL > select to_char(to_date(‘29-Mar-2013’,’dd-mon-yyyy’),’J’) as julian from dual;
Spool command can print the output of sql statements in a file.
select smp_name, smp_id from smp where dept=’accounts’;
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.
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.
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.
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.
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.
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.
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
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
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.
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
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.
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.
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.
The Description column of the USERJTRIGGERS view combines information from many columns to display the trigger header, which includes the database event.
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.
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.
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.
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.
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.
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.
No, DDL triggers are not executed for DDL statements, which are executed within the PL/SQL code using the DBMS_SQL package.
Yes, the USER_OBJECTS view has one row entry for each trigger in the schema.
The USERJERRORS view can be used to show all the parsing errors that occur in a trigger during the compilation until they are resolved.
Yes, USER_TRIGGERS have entries for all triggers that are created in the schema with or without errors.
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.
No, a SELECT statement cannot fire a trigger. DML statements, such as INSERT, UPDATE, and DELETE, can cause triggers to fire.
Yes, cursors can be a part of code in trigger.
No, It is not possible to create STARTUP or SHUTDOWN triggers for ON-SCHEMA.
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.
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.
No, INSTEAD OF triggers cannot be used for each statement however, It can only be used for each row on a view.
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.
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.
The ALTER statement is given as follows:
ALTER TABLE T_STUDENTS ENABLE ALL TRIGGERS;
The ACTION_TYPE column of the USER_TRIGGERS data dictionary view shows that the trigger is a PL/SQL trigger.