count(XXX=xxx or null) 相当于 count(case when XXX=xxx then ... else null end) ,好理解点
2017-08-25
追加 or null 的原因找到了:
https://stackoverflow.com/questions/5011239/why-do-i-need-or-null-in-mysql-when-counting-rows-with-a-condition
https://stackoverflow.com/questions/5011239/why-do-i-need-or-null-in-mysql-when-counting-rows-with-a-condition
2017-08-19