查询对我来说不可能？

Question

我有2列a和b与1：n关系：

A.id, a.text, a.b_id(fk), a.value --
B.id(pk), b.text etc

我想创建一个返回b.id count(a.value=1) > 0

的数量的查询

我试过了：

Select count(b.id)
from a.id
join b on a.b_id=b_id
group by b.id
having count(if(a.value=1),1,null))>0

......但没有结果。看起来很简单，但对我来说是一个问题。

Answer 1

您不需要HAVING，因为无论如何，标准INNER联接都不会返回b行而不会匹配a行。您也不需要GROUP BY - 使用COUNT(DISTINCT )代替：

SELECT COUNT(DISTINCT b.id) AS cnt
FROM b
JOIN a ON a.b_id = b.id
WHERE a.value = 1

Answer 2

这应该做：

SELECT COUNT(*)
FROM TableB AS B
WHERE EXISTS(SELECT 1 FROM TableA
             WHERE id = B.id
             AND value = 1)

Answer 3

试试这个： -

Select count(b.id) as x
FROM b
JOIN a ON a.b_id = b.id
WHERE a.value = 1
group by b.id

Answer 4

@MarcinJuraszek's answer的简化。如果可以信任外键，则无需加入表b：

SELECT COUNT(DISTINCT a.b_id) AS cnt
FROM a
WHERE a.value = 1 ;