mysql - 如何外部连接两个 mysql 查询？

Question

假设有 4 个表，这样 t1.id = t2.pid 和 t3.id = t4.pid

我如何在哪里获取记录t1.val1 != t3.val1 AND t2.val2 != t4.val2？

Answer 1

您需要使用LEFT OUTER JOIN从两个表中查找不匹配的记录。

SELECT *
FROM t1
     INNER JOIN t2
           ON t1.id = t2.pid
     LEFT OUTER JOIN t3
           ON t2.pid = t3.id
     LEFT OUTER JOIN t4
           ON t3.id = t4.pid
WHERE (t1.val1 IS NULL OR t3.val1 IS NULL)
      AND (t2.val2 IS NULL OR t4.val2 IS NULL)

访问： http: //www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html