sql-帮助将子查询转换为具有联接的查询

我陷入了联接查询.客户端站点正在运行mysql4,因此不能选择子查询.我使用联接重写的尝试进行得不太顺利.

我需要选择承包商表中列出的所有不在给定标签ID&县ID.但是,它们可能会列在
contractors2label,以及其他标签和县ID.

表：承包商

cID (primary, autonumber)
company (varchar)
…etc…

表：contractors2label

cID
labelID
countyID
psID

此查询与子查询一起工作：

SELECT company, contractors.cID
   FROM contractors
   WHERE contractors.complete = 1
   AND contractors.archived = 0
   AND contractors.cID NOT IN (
     SELECT contractors2label.cID FROM contractors2label
        WHERE labelID <> 1 AND countyID <> 1
   )

我认为带有联接的查询将是等效的,但它不会返回任何结果.手动扫描数据显示我应该得到34行,这是上面的子查询返回的结果.

SELECT company, contractors.cID
   FROM contractors 
   LEFT OUTER JOIN contractors2label ON contractors.cID = contractors2label.cID
   WHERE contractors.complete = 1
   AND contractors.archived = 0
   AND contractors2label.labelID <> 1
   AND contractors2label.countyID <> 1
   AND contractors2label.cID IS NULL

解决方法:

进行LEFT JOIN时,需要将JOIN的所有条件放入ON子句中.

在您的示例中,对于不存在的左连接列,您将得到NULL,但是您随后将它们再次与无效的值(<> 1)进行比较.

SELECT c.company, c.cID
   FROM contractors c
   LEFT JOIN contractors2label c2
          ON ( c2.cID = c.cID AND c2.labelID <> 1 AND c2.countyID <> 1 )
   WHERE c.complete = 1
   AND c.archived = 0
   AND c2.cID IS NULL

顺便说一句：使用别名(例如我的示例中的c)使读取和写入查询更加容易.

标签：left-join,sql,mysql
来源： https://codeday.me/bug/20191106/2000862.html