MySQL COUNT()和null

本文介绍了MySQL COUNT()和null的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我是否正确地说:

COUNT(expr) WHERE expr IS NOT *

仅计算非null吗?

Will count only non nulls?

COUNT(*)始终计数所有行吗?而且如果所有列都为空怎么办?

Will COUNT(*) always count all rows? And What if all columns are null?

推荐答案

正确. COUNT(*)是表中的所有行，COUNT(Expression)是表达式仅非空的地方.

Correct. COUNT(*) is all rows in the table, COUNT(Expression) is where the expression is non-null only.

如果所有列均为NULL(表示您没有主键，那么这在规范化数据库中就不会发生)COUNT(*) still 返回所有插入的行.只是不要那样做.

If all columns are NULL (which indicates you don't have a primary key, so this shouldn't happen in a normalized database) COUNT(*) still returns all of the rows inserted. Just don't do that.

您可以将*符号认为是在表格中"而不是在任何列中".

You can think of the * symbol as meaning "in the table" and not "in any column".

MySQL参考手册对此进行了介绍.