涉及多个列的子查询

Question

我希望查询返回另一个表中不存在的值。 我目前运行两个查询，并在代码中进行交集。 我被困在多列的语法和where之后的语句的存在

第一个查询：

SELECT sid, cid 
FROM  Table2
where used = 0
group by sid, cid

主要查询：

SELECT    sid, cid, count(1) as cnt 
FROM      Table1
WHERE     ##not any pair of (sid, cid) returned from first query##
GROUP BY  sid, cid 
HAVING    cnt < 20 
LIMIT     50

什么是完整的主要查询？

Answer 1

尝试：

SELECT    t1.sid, t1.cid, count(1) as cnt 
FROM      Table1 t1
LEFT JOIN Table2 t2
ON        t1.sid = t2.sid AND t1.cid = t2.cid AND t2.used = 0
WHERE     t2.sid IS NULL AND t2.cid IS NULL
GROUP BY  sid, cid 
HAVING    cnt < 20 
LIMIT     50