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.
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MySQL COUNT()和null
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