sql中的case when语句

1.在where子句中：

 CREATE  TABLE `hello`.`sometbl` (

   `id` INT NOT NULL AUTO_INCREMENT ,

   `a` VARCHAR(45) NULL ,

   `b` VARCHAR(45) NULL ,

   `c` VARCHAR(45) NULL ,

   PRIMARY KEY (`id`) ,

   UNIQUE INDEX `id_UNIQUE` (`id` ASC) );

 INSERT INTO `hello`.`sometbl` (`a`, `b`, `c`) VALUES ('', '', '');

 INSERT INTO `hello`.`sometbl` (`b`, `c`) VALUES ('', '');

 INSERT INTO `hello`.`sometbl` (`a`, `c`) VALUES ('', '');

 INSERT INTO `hello`.`sometbl` (`b`) VALUES ('');

查询a,b,c中有两个满足=1条件的记录

SELECT * FROM hello.sometbl where ((case when a='' then 1 else 0 end)
+(case when b='' then 1 else 0 end)
+(case when c='' then 1 else 0 end))=2;

2。在select中作为一列

id为1,2,3,4时分别输出，一，二，三，四

SELECT id,case when id=1 then '一'
when id=2 then '二'
when id=3 then '三'
when id=4 then '四'
else '其它' end 中文数字
FROM hello.sometbl;

输出