SQL语句语法？ [英] SQL Case Statement Syntax?

问题描述

推荐答案

对于SQL Server：

For SQL Server:

CASE case-expression
    WHEN when-expression-1 THEN value-1
  [ WHEN when-expression-n THEN value-n ... ]
  [ ELSE else-value ]
END

或：

CASE
    WHEN boolean-when-expression-1 THEN value-1
  [ WHEN boolean-when-expression-n THEN value-n ... ]
  [ ELSE else-value ]
END

表达式等：

case-expression    - something that produces a value
when-expression-x  - something that is compared against the case-expression
value-1            - the result of the CASE statement if:
                         the when-expression == case-expression
                      OR the boolean-when-expression == TRUE
boolean-when-exp.. - something that produces a TRUE/FALSE answer

链接： CASE（Transact-SQL）

还要注意WHEN语句的顺序很重要。您可以轻松编写多个重叠的WHEN子句，匹配的第一个。

Also note that the ordering of the WHEN statements is important. You can easily write multiple WHEN clauses that overlap, and the first one that matches is used.

注意：如果未指定ELSE子句，并且未找到匹配的WHEN条件，则CASE表达式的值将为 NULL 。

Note: If no ELSE clause is specified, and no matching WHEN-condition is found, the value of the CASE expression will be NULL.

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