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在執(zhí)行 Transact-SQL 語句時強(qiáng)加條件。如果 Boolean_expression 取值為 TRUE，則執(zhí)行 Boolean_expression 后的 Transact-SQL 語句 (sql_statement)。當(dāng) Boolean_expression 取值為 FALSE 或 NULL時，則可選 ELSE 關(guān)鍵字為要執(zhí)行的 Transact-SQL 語句。

語法

IF Boolean_expression { sql_statement | statement_block }

[

    ELSE

    { sql_statement | statement_block } ]

參數(shù)

Boolean_expression

返回 TRUE 或 FALSE 的表達(dá)式。如果布爾表達(dá)式中含有 SELECT 語句，必須用圓括號將 SELECT 語句括起來。

{sql_statement | statement_block}

是任何有效的 Transact-SQL 語句或以語句塊定義的語句分組。若要定義語句塊（批處理），采用控制流語言關(guān)鍵字 BEGIN 和 END。雖然所有的 Transact-SQL 語句在 BEGIN...END 塊內(nèi)都有效，但有些 Transact-SQL 語句不應(yīng)組合在同一個批處理（語句塊）中。

結(jié)果類型

Boolean

示例

當(dāng)至少一本書滿足這些條件時，這個示例會給出價格在 $10 與 $20 之間的傳統(tǒng)烹調(diào)書的列表。否則，SQL Server 會給出一條信息，說明沒有書滿足這個條件，并生成價格低于 $10 的傳統(tǒng)烹調(diào)書的列表。

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

下面是結(jié)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)