MySQL:UPDATE表与另一个表中的COUNT? [英] MySQL: UPDATE table with COUNT from another table?

问题描述

我以为这很简单，但是我无法理解...

I thought this would be simple but I can't get my head around it...

我有一个表tbl1，它具有列id，otherstuff，num.

I have one table tbl1 and it has columns id,otherstuff,num.

我有另一个表tbl2，它具有列id，info.

I have another table tbl2 and it has columns id,info.

我要使tbl1的num列等于tbl2中具有相同id的行数.有点像这样:

What I want to is make the num column of tbl1 equal to the number of rows with the same id in tbl2. Kind of like this:

UPDATE tbl1 SET num =
(SELECT COUNT(*) FROM tbl2 WHERE id=tbl1.id)

有什么想法吗?

推荐答案

如果您的num列是有效的数字类型，则您的查询应按原样工作:

If your num column is a valid numeric type your query should work as is:

UPDATE tbl1 SET num = (SELECT COUNT(*) FROM tbl2 WHERE id=tbl1.id)

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