Introduction to. NET Foundation--mssql functions IsNull ()

ISNULL ()

Replaces null with the specified replacement value.

Grammar

ISNULL (Check_expression, Replacement_value)

Parameters

Check_expression

An expression that will be checked for null. Check_expression can be of any type.

Replacement_value

The expression that will be returned when check_expression is null. Replacement_value must have the same type as check_expresssion.

return type

Returns the same type as check_expression.

Comments

If Check_expression is not NULL, the value of the expression is returned, otherwise Replacement_value is returned.

Example

A. Using ISNULL with AVG

The following example finds the average price for all books and replaces all null entries in the Value column of the titles table with the values $10.00.

Usepubs

GO

Selectavg (ISNULL (Price, $10.00))

Fromtitles

GO

here is the result set :

--------------------------

14.24

(1row (s) affected)

B. Using ISNULL

The following example selects the title, type, and price for all books in the titles table. If the price of a title is NULL, then the price shown in the result set is 0.00.

Usepubs

GO

Selectsubstring (title, 1,) as title, type as type,

ISNULL (Price, 0.00) AS-price

Fromtitles

GO

C. Using IsNull in full join case

Table A:

Tid

Uid

Anum

Table B:

Tbid

Uid

Bnum1

Bnum2

All two tables need to be connected by UID:

Selecta.tid,a.uid,a.anum,b.bnum1,b.bnum2 from a full join B ona.uid=b.uid

The full connection will be a lot of empty cases, you can use IsNull to solve, instead:

Selectisnull (A.tid,b.tid), IsNull (A.uid,b.uid), IsNull (a.anum,0), IsNull (B.BNUM1,

Summary: The most important feature of this function is to assign a default value to a null value, to avoid the occurrence of null values for non-null properties!

Introduction to. NET Foundation--mssql functions IsNull ()