case when… then… else… end：
CASE 字段 WHEN 预期值 THEN 结果1 ELSE 结果2 END
score 表：
mysql> select * from score;
+----+----------+------+------+-------+
| id | name | sex | age | score |
+----+----------+------+------+-------+
| 1 | zhangsan | 1 | 19 | 99 |
| 2 | lisi | 0 | 25 | 89 |
| 3 | wangwu | 1 | 21 | 79 |
| 4 | zhaoliu | 0 | 18 | 69 |
| 5 | sunqi | 1 | 23 | 59 |
| 6 | zhouba | 1 | 15 | 49 |
| 7 | wujiu | 0 | 20 | 39 |
+----+----------+------+------+-------+
示例： 查找男女，以汉字显示
1、使用 case when ，符合要求的返回 1，case when 外部用 sum 进行求和
select
name,
(case sex when '1' then '男' else '女' end) sex
from score;
if 函数
select name, if(sex = 1, '男', '女') as sex from score;
+----------+-----+
| name | sex |
+----------+-----+
| zhangsan | 男 |
| lisi | 女 |
| wangwu | 男 |
| zhaoliu | 女 |
| sunqi | 男 |
| zhouba | 男 |
| wujiu | 女 |
+----------+-----+
条件表达式：
CASE
WHEN condition THEN result1 ELSE result2
END
如果没有 end，返回 NULL
注意： 多个 when 之间，不必加逗号
select
name,
score,
(case
when score > 90 then 'nice'
when score > 80 then 'good'
when score >= 60 then'not bad'
else 'bad' end) as level
from score;
示例 2： 统计总人数，男女人数和男女成年的人数
1、case when 外部用 sum 进行求和
select
count(id) as '总人数',
sum(case when sex = '1' then 1 else 0 end) as '男生人数',
sum(case when sex = '0' then 1 else 0 end) as '女生人数',
sum(case when age >= 18 and sex = '1' then 1 else 0 end) as '成年男生人数',
sum(case when age >= 18 and sex = '0' then 1 else 0 end) as '成年女生人数'
from score;
if 函数计数
select
count(if(age >= 18 and sex = 1, 1, null)) as '成年男生人数',
count(if(age >= 18 and sex = 0, 1, null)) as '成年女生人数'
from score;
+-----------+--------------+--------------+--------------------+--------------------+
| 总人数 | 男生人数 | 女生人数 | 成年男生人数 | 成年女生人数 |
+-----------+--------------+--------------+--------------------+--------------------+
| 7 | 4 | 3 | 3 | 3 |
+-----------+--------------+--------------+--------------------+--------------------+