在执行 Transact-SQL 语句时强加条件。如果 Boolean_expression 取值为 TRUE，则执行 Boolean_expression 后的 Transact-SQL 语句 (sql_statement)。当 Boolean_expression 取值为 FALSE 或 NULL时，则可选 ELSE 关键字为要执行的 Transact-SQL 语句。

语法

IF Boolean_expression { sql_statement | statement_block }

[

    ELSE

    { sql_statement | statement_block } ]

参数

Boolean_expression

返回 TRUE 或 FALSE 的表达式。如果布尔表达式中含有 SELECT 语句，必须用圆括号将 SELECT 语句括起来。

{sql_statement | statement_block}

是任何有效的 Transact-SQL 语句或以语句块定义的语句分组。若要定义语句块（批处理），采用控制流语言关键字 BEGIN 和 END。虽然所有的 Transact-SQL 语句在 BEGIN...END 块内都有效，但有些 Transact-SQL 语句不应组合在同一个批处理（语句块）中。

结果类型

Boolean

示例

当至少一本书满足这些条件时，这个示例会给出价格在 $10 与 $20 之间的传统烹调书的列表。否则，SQL Server 会给出一条信息，说明没有书满足这个条件，并生成价格低于 $10 的传统烹调书的列表。

USE pubs
GO
DECLARE @msg varchar(255)
IF (SELECT COUNT(price)
   FROM titles 
   WHERE title_id LIKE 'TC%' AND price BETWEEN 10 AND 20) > 0
   
   BEGIN
     SET NOCOUNT ON
     SET @msg = 'There are several books that are a good value between $10 and $20. These books are: '
         PRINT @msg
      SELECT title 
     FROM titles 
     WHERE title_id LIKE 'TC%' AND price BETWEEN 10 AND 20
   END
ELSE
   BEGIN
     SET NOCOUNT ON
     SET @msg = 'There are no books between $10 and $20. You might consider the following books that are under $10.'
         PRINT @msg
     SELECT title 
     FROM titles 
     WHERE title_id LIKE 'TC%' AND price < 10       
   END

下面是结果集：

There are several books that are a good value between $10 and $20. These books are: 
title                                                                   
------------------------------------------------------------------------
Fifty Years in Buckingham Palace Kitchens
Sushi, Anyone?

(2 row(s) affected)