MySQL - 5.1 Reference Manual参考手册 - 12.8.6.2. CASE Statement - CASE语句

CASE case_value
    WHEN when_value THEN statement_list
    [WHEN when_value THEN statement_list] ...
    [ELSE statement_list]
END CASE

Or:

CASE
    WHEN search_condition THEN statement_list
    [WHEN search_condition THEN statement_list] ...
    [ELSE statement_list]
END CASE

The CASE statement for stored programs implements a complex conditional construct. If a search_condition evaluates to true, the corresponding SQL statement list is executed. If no search condition matches, the statement list in the ELSE clause is executed. Each statement_list consists of one or more statements.

If no when_value or search_condition matches the value tested and the CASE statement contains no ELSE clause, a Case not found for CASE statement error results.

Each statement_list consists of one or more statements; an empty statement_list is not allowed. To handle situations where no value is matched by any WHEN clause, use an ELSE containing an empty BEGIN ... END block, as shown in this example:

DELIMITER |

CREATE PROCEDURE p()
  BEGIN
    DECLARE v INT DEFAULT 1;

    CASE v
      WHEN 2 THEN SELECT v;
      WHEN 3 THEN SELECT 0;
      ELSE
        BEGIN
        END;
    END CASE;
  END;
  |

(The indentation used here in the ELSE clause is for purposes of clarity only, and is not otherwise significant.)

Note

The syntax of the CASE statement used inside stored programs differs slightly from that of the SQL CASE expression described in Section11.3, “Control Flow Functions”. The CASE statement cannot have an ELSE NULL clause, and it is terminated with END CASE instead of END.

[MySQL - 5.1 Reference Manual 参考手册 - 12 SQL Statement Syntax - SQL语句语法 - 12.8 MySQL Compound-Statement Syntax - MySQL复合语句语法 - 12.8.6 Flow Control Constructs流程控制结构 -12.8.6.2. CASE Statement - CASE语句 - 文档]

http://dev.mysql.com/doc/refman/5.1/en/case-statement.html

[MySQL - 5.1 Reference Manual参考手册]

MySQL - 5. 数据库管理 - 5.3. mysqld：MySQL服务器 - 5.3.3. 服务器系统变量 - max_allowed_packet, http://dev.mysql.com/doc/refman/5.1/zh/database-administration.html#server-system-variables

9. 语言结构 - 9.2. 数据库、表、索引、列和别名 - 9.2.2. 识别符大小写敏感性, http://dev.mysql.com/doc/refman/5.1/zh/language-structure.html#name-case-sensitivity
13. SQL语句语法 - 13.1.数据定义语句 - 13.1.5.CREATE TABLE语法, http://dev.mysql.com/doc/refman/5.1/zh/sql-syntax.html#create-table
12 SQL Statement Syntax - SQL语句语法 - 12.8 MySQL Compound-Statement Syntax - MySQL复合语句语法 - 12.8.6 Flow Control Constructs流程控制结构 -12.8.6.2. CASE Statement - CASE语句, http://dev.mysql.com/doc/refman/5.1/en/case-statement.html