涉及多列的子查詢

Question

我想要一個查詢來返回不存在於另一個表中的值。我目前運行兩個查詢並在代碼中進行交集。我堅持的語法，多列和語句的存在之後where

首先查詢：

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

主查詢：

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 

什麼是完整的主查詢？

Answer 1

嘗試：

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