1.Mysql的if作为表达式用

IF(expr1,expr2,expr3)

含义：如果 expr1 是TRUE (expr1 <> 0 and expr1 <> NULL)，则 IF()的返回值为expr2; 否则返回值则为 expr3。

mysql> select *,if(sex=1,"男","女") as new_sex  from student where sex != "";
+----+------+--------+---------+
| id | sex  | name   | new_sex |
+----+------+--------+---------+
|  1 |    1 | 张三   | 男      |
|  2 |    2 | 翠花   | 女      |
|  3 |    1 | 李四   | 男      |
+----+------+--------+---------+
3 rows in set (0.00 sec)

备注:当翠花的性别为0时，即便 sex != “”，也查询不出来；不知何故；

等同于

mysql> select *, CASE sex WHEN 1 THEN '男' ELSE '女' END as new_sex from student where sex != '';
+----+------+--------+---------+
| id | sex  | name   | new_sex |
+----+------+--------+---------+
|  1 |    1 | 张三   | 男      |
|  2 |    2 | 翠花   | 女      |
|  3 |    1 | 李四   | 男      |
+----+------+--------+---------+
3 rows in set (0.00 sec)

2.存储过程中作为流程控制语句使用

IF ELSE 做为流程控制语句使用

IF search_condition THEN 
    statement_list  
[ELSEIF search_condition THEN]  
    statement_list ...  
[ELSE 
    statement_list]  
END IF 

备注:了解即可；

3.实战

1.需求：有这样一个表，需要求出第一列大于3的个数，第二列大于3的个数；

表信息如下：

mysql> select * from data;
+-----------+-----------+
| data_co_1 | data_co_2 |
+-----------+-----------+
|      1.00 |      0.50 |
|      0.90 |      1.50 |
|      3.00 |      0.70 |
|      4.00 |      3.00 |
|      0.01 |     80.00 |
|    100.00 |      0.92 |
|      0.70 |      0.25 |
+-----------+-----------+
7 rows in set (0.00 sec)

2.方案1 ，使用sum(if()) ;

mysql> select sum(if (data_co_1 > 3,1,0)) count1,sum(if (data_co_2 > 3,1,0)) count2 from data;
+--------+--------+
| count1 | count2 |
+--------+--------+
|      2 |      1 |
+--------+--------+
1 row in set (0.00 sec)

2.2.方案2 ，使用sum(case when) ;

mysql> select sum(case when data_co_1 >3 then 1 else 0 end) count1,sum(case when data_co_2 >3 then 1 else 0 end) count2 from data;
+--------+--------+
| count1 | count2 |
+--------+--------+
|      2 |      1 |
+--------+--------+
1 row in set (0.00 sec)