示例

表数据

统计错误的sql

select count(age = 10) as count from student

查询结果

原因

count(‘任意内容’)都会统计出所有记录数，因为count只有在遇见null时才不计数，即count(null)==0

解决

方法1：count()函数中条件表达式加上or null

select count(age = 10 or null) as count from student

方法2：使用IF()函数，当不满足条件时表达式的值为NULL

select count(IF(age = 10,1,null)) as count from student

方法3：使用case when表达式，当不满足条件时表达式的值为NULL

select count(case when age = 10 then 1 else null end) as count from student