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场景

我们经常在数据存储如性别、类型此类的字段，一般使用tinyint来处理，那么展示的时候就需要用相应的文字值去展示，可以通过程序或者sql直接处理。对于一些类型字段如果数据库没有给默认值，那可能存入的是null，而有些却有值，那这种又该如何处理。

语法

if(expr,v1,v2) 如果expr为真则返回v1,否则返回v2
isnull(expr) 判断expr是否为null
ifnull(v1,v2) 如果v1存在则返回v1,否则返回v2

使用

--展示对应性别 if
select id,nickname,if(gender=1, '男', '女') as gender from user where id = 1565543;

+---------+-----------+--------+
| id      | nickname  | gender |
+---------+-----------+--------+
| 1565543 | 马小桶    | 男     |
+---------+-----------+--------+
1 row in set (0.00 sec)

--ifnull
select id, email, gender from user where id =5515;
+------+-------+--------+
| id   | email | gender |
+------+-------+--------+
| 5515 | NULL  |      2 |
+------+-------+--------+
1 row in set (0.00 sec)


select id,ifnull(email, '暂无邮箱') as email from user where id = 5515
+------+--------------+
| id   | email        |
+------+--------------+
| 5515 | 暂无邮箱     |
+------+--------------+
1 row in set (0.00 sec)

--isnull

select id, if(isnull(email), '暂无邮箱', email) as email from user where id = 5515;
+------+--------------+
| id   | email        |
+------+--------------+
| 5515 | 暂无邮箱     |
+------+--------------+
1 row in set (0.01 sec)


select id, if(isnull(email), '暂无邮箱', email) as email from user where id = 1561977;
+---------+---------------------+
| id      | email               |
+---------+---------------------+
| 1561977 | [email protected] |
+---------+---------------------+
1 row in set (0.00 sec)