我们如何在mysql中实现交集和并集？

Question

So i am trying to Find the sids of suppliers who supply a red part and a green part.

Here is SQL query

SELECT DISTINCT C.sid
FROM Catalog C, Parts P
WHERE C.pid = P.pid AND P.color  = 'red'
INTERSECT 
(   SELECT DISTINCT C.sid
    FROM Catalog C1,Parts P1
    WHERE C1.pid = P1.pid AND P1.color = 'green'
);

but i have a hard time implementing it in mysql , any suggestions appreciated

Answer 1

Just use two derived tables:

 select
    t1.sid
    from
    (SELECT DISTINCT C.sid FROM Catalog C, Parts P WHERE C.pid = P.pid AND P.color = 'red') t1
    inner join
    (SELECT DISTINCT C.sid FROM Catalog C,Parts P WHERE C.pid = P.pid AND P.color = 'green') T2
    on t1.sid = t2.sid

(I just threw this together but it should at least be close)

Answer 2

Why to go for such complex? Try this, may this work for your need

SELECT DISTINCT C.sid
FROM Catalog C, Parts P
WHERE C.pid = P.pid AND P.color  = 'red' AND P.color = 'green'

This will give you join both the tables and will SELECT rows where color is either red and green . Kind of intersect operation, which you wish to apply.

As far as concept is concerned, I would suggest ( or what I know ) UNION and INTERSECTION should be applied when dealing with different tables. It should not be practiced for operation over the same table.

Suppose you have two tables A and B , then you can should apply UNION and INTERSECTION not on the same table. That is, If you are querying the same table A , and over same column then you can go for AND or OR operators.

Making a note of the above, you can achieve this by doing AND .