Oracle DBA FAQ - Understanding PL/SQL Language Basics

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What Is NULL in PL/SQL?

NULL is a reserved key word and it stands for two things in PL/SQL:

NULL is an executable statement, and means doing nothing.
NULL is a data balue, and means no value.

The following sample script shows you examples of using NULL keyword:

DECLARE
  next_task CHAR(80);
BEGIN
  next_task := NULL; -- NULL value

  IF next_task IS NOT NULL THEN
     DBMS_OUTPUT.PUT_LINE('I am busy.');
  ELSE
     DBMS_OUTPUT.PUT_LINE('I am free.');
  END IF;
  
  IF next_task IS NULL THEN
     NULL; -- NULL statement
  ELSE
     DBMS_OUTPUT.PUT_LINE('... working on ' || next_task);
  END IF;
END;

This script should print this:

I am free.

How To Test NULL Values?

There ate two special comparison operators you can use on NULL values:

"variable IS NULL" - Returns TRUE if the variable value is NULL.
"variable IS NOT NULL" - Return TRUE if the variable value is not NULL.

The following sample script shows you examples of comparing NULL values:

DECLARE
  next_task CHAR(80);
BEGIN
  next_task := NULL;

  IF next_task IS NOT NULL THEN
     DBMS_OUTPUT.PUT_LINE('I am busy.');
  ELSE
     DBMS_OUTPUT.PUT_LINE('I am free.');
  END IF;
  
  IF next_task IS NULL THEN
     NULL;
  ELSE
     DBMS_OUTPUT.PUT_LINE('... working on ' || next_task);
  END IF;
END;

Note that "variable = NULL" is not a valid operation. This script should print this:

I am free.

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