Mysql中count()函数的一般用法是统计字段非空的记录数,所以可以利用这个特点来进行条件统计,注意这里如果字段是NULL就不会统计,但是false是会被统计到的,记住这一点,我们接下来看看几种常见的条件统计写法。
测试环境
Windows 10
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准备工作
新建一个Mysql数据表a,包含id和num两个字段
mysql> create table a(id int, num int);
Query OK, 0 rows affected (0.04 sec)
插入测试数据,为了看count()函数的效果,我们插入两个空数据
mysql> insert into a values (1,100),(2,200),(3,300),(4,300),(8,null),(9,null);
Query OK, 6 rows affected (0.01 sec)
Records: 6 Duplicates: 0 Warnings: 0
查询表a中的数据,与后面的统计做比较
mysql> select * from a;
| id | num |
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
| 4 | 300 |
| 8 | NULL |
| 9 | NULL |
6 rows in set (0.09 sec)
调用count()函数看效果,如果使用count(*)会查询出所有的记录数,但如果使用count(num)发现只有4条数据,num为NULL的记录并没有统计上
mysql> select count(*) from a;
| count(*) |
| 6 |
1 row in set (0.03 sec)
mysql> select count(num) from a;
| count(num) |
| 4 |
1 row in set (0.04 sec)
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count()函数中使用条件表达式加or null来实现,作用就是当条件不满足时,函数变成了count(null)不会统计数量
mysql> select count(num > 200 or null) from a;
| count(num > 200 or null) |
| 2 |
1 row in set (0.22 sec)
count()函数中使用if表达式来实现,当条件满足是表达式的值为非空,条件不满足时表达式值为NULL;
mysql> select count(if(num > 200, 1, null)) from a;
| count(if(num > 200, 1, null)) |
| 2 |
1 row in set (0.05 sec)
count()函数中使用case when表达式来实现,当条件满足是表达式的结果为非空,条件不满足时无结果默认为NULL;
mysql> select count(case when num > 200 then 1 end) from a;
| count(case when num > 200 then 1 end) |
| 2 |
1 row in set (0.07 sec)
总结
使用count()函数实现条件统计的基础是对于值为NULL的记录不计数,常用的有以下三种方式,假设统计num大于200的记录
select count(num > 200 or null) from a;
select count(if(num > 200, 1, null)) from a
select count(case when num > 200 then 1 end) from a