在写sql语句时，遇到比较复杂的sql可能经常会用到CASE WHEN判断，CASE WHEN的基本语法在此不再赘述，网上有许多相关教程。
数据准备：
DROP TABLE IF EXISTS `t_user`;
CREATE TABLE `t_user` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(10) NOT NULL,
`sex` smallint(1) NOT NULL,
`email` varchar(40) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=13 DEFAULT CHARSET=utf8;
INSERT INTO t_user(name, sex, email) VALUES ('zong', '0', 'zong@163.com');
INSERT INTO t_user(name, sex, email) VALUES ('liu', '1', null);
INSERT INTO t_user(name, sex, email) VALUES ('ma', '1', null);
INSERT INTO t_user(name, sex, email) VALUES ('wang', '0', 'wang@163.com');
INSERT INTO t_user(name, sex, email) VALUES ('zhao', '0', null);
INSERT INTO t_user(name, sex, email) VALUES ('li', '1', null);
简单使用：
SELECT name, CASE sex WHEN 0 THEN '男' WHEN 1 THEN '女' ELSE '' END FROM t_user;
SELECT name, CASE sex WHEN 0 THEN '男' WHEN 1 THEN '女' ELSE '' END sex FROM t_user;
判断email是否为null：
SELECT
name,
CASE WHEN email IS NULL THEN '' ELSE email END email
FROM t_user;
