MS SQL Server Interview Question:

How To Replace NULL Values in Expressions using ISNULL()?

Submitted by: Administrator

As you learned from provious tutorials, NULL values presented in expressions will cause the final results to be NULL. Sometimes, you want NULL values to be replaced with some default values, like 0, '', or 'NULL', so that expressions can be evaluated properly.

SQL Server offers a built-in function called ISNULL() to help you replacing NULL values in expressions:

ISNULL(expression, replacement)
-- Returns "expression", if it is not NULL
-- Returns "replacement", if "expression" is NULL

The tutorial example below shows you how to replace possible NULL values in @middle_initial:

USE GlobalGuideLineDatabase;
GO

CREATE PROCEDURE welcome
@first_name VARCHAR(20),
@middle_initial VARCHAR(1),
@last_name VARCHAR(20)
AS
PRINT 'Hello '+@first_name
+ ' '+@middle_initial
+ ' '+@last_name;
PRINT 'Hello '+@first_name
+ ' '+ISNULL(@middle_initial,'')
+ ' '+@last_name;
GO

EXEC welcome 'John', 'W', 'King';
GO
Hello John W King
Hello John W King

EXEC welcome 'John', NULL, 'King';
GO

Hello John King

The first PRINT statement in the second test returns a blank line becaus
Submitted by: Administrator