MySQL：如何在给定其他某个列值的情况下选择其中两个列值的组合唯一的行

Question

我有一个像表(id,C1,C2,R,..)表，在此表中， C1-C2对与一个或多个R ，例如(id1,a,b,r1,...) (id2,a,b,r2,...)我想找出所有仅与某个R相关的C1-C2对（例如对于r1 ，则不包括ab对，因为它也与r2 ），我应该如何编写查询？

Answer 1

You could group by C1 and C2 and count the number of "r"s that are equal to the value you want (and make sure it's not 0) and the number of "r"s that aren't (and make sure it is 0):

SELECT   c1, c2
FROM     mytbale
GROUP BY c1, c2
HAVING   COUNT(CASE r WHEN 'r1' THEN e END) > 0 AND
         COUNT(CASE r WHEN 'r1' THEN NULL ELSE 1 END) = 0 

Answer 2

If I understand correctly, you can use group by and having . I you want all pairs that are only related to one r :

select c1, c2, max(r) as r
from t
group by c1, c2
having min(r) = max(r);

If you have a particular r in mind:

select c1, c2
from t
group by c1, c2
having min(r) = max(r) and min(r) = 'r1'