Question

我的查询是：

select * from A left join B on a.id = b.id 
and a. id <> "N/A" and b.id <> "N/A" 
OR 
A.ID = "N/A" AND B.ID = "N/A" AND A.COL2 IS NOT NULL

基本上，只有在满足其中一个id条件时，我才想加入OR列。
这个查询是正确的还是我需要在括号中包含该部分？

a. id <> "N/A" and b.id "N/A" 
OR 
A.ID = "N/A" AND B.ID = "N/A" AND A.COL2 IS NOT NULL

Answer 1

AND的优先级高于OR（类似于＆＃34; *来自+＆＃34;）。

因此这个表达式：

a. id <> "N/A" AND b.id <> "N/A" 
OR 
A.ID = "N/A" AND B.ID = "N/A" AND A.COL2 IS NOT NULL

......等同于......

(a. id <> "N/A" AND b.id <> "N/A")
OR 
(A.ID = "N/A" AND B.ID = "N/A" AND A.COL2 IS NOT NULL)

但是因为a.id = b.id应该永远是真的，你必须至少放一对括号

SELECT * FROM A LEFT JOIN B ON 
  a.id = b.id AND 
  (
    a. id <> "N/A" AND b.id <> "N/A" 
    OR 
    A.ID = "N/A" AND B.ID = "N/A" AND A.COL2 IS NOT NULL
  )

带AND OR条件的左连接需要括在括号中

1 个答案: