MySQL 将两个查询组合成一个查询答案

【问题标题】：MySQL combining two queries into single queryMySQL 将两个查询组合成一个查询
【发布时间】：2018-08-07 23:27:57
【问题描述】：

我的第一个查询如下所示

SELECT a.name, b.desc, T3.desc1 as Output
FROM table1 a INNER JOIN
     table2 b
     ON a.id = b.id
INNER JOIN (
  SELECT b2.id, b2.status,  MAX(b2.created_time) as max_time
  FROM q_scan b2
  GROUP BY b2.id  ,  b2.qualys_type
  ) t on t.id = a.id
      AND  t.status=b.status
      AND t.max_time = b.created_time
INNER JOIN table3 T3 on b.tid = T3.tid
WHERE b.status = 'FAIL'

我的第二个查询如下所示

SELECT a.name, b.desc, T4.desc2 as Output
FROM table1 a INNER JOIN
     table2 b
     ON a.id = b.id
INNER JOIN (
  SELECT b2.id, b2.status,  MAX(b2.created_time) as max_time
  FROM q_scan b2
  GROUP BY b2.id  ,  b2.qualys_type
  ) t on t.id = a.id
      AND  t.status=b.status
      AND t.max_time = b.created_time
INNER JOIN table4 T4 on b.tid = T4.tid
WHERE b.status = 'FAIL'

在我的最终结果中，我想要Output column，它的值来自T3.desc1 或T4.desc2 如何将两个查询合并为一个查询？

【问题讨论】：

select ... t1 ... t2 ... (t3 union all t4) ...?

标签： mysql sql join union

【解决方案1】：

如果table2.id 存在于其中一个表中——table3 或table4，LEFT JOIN 将在IFNULL() 的帮助下简化查询。

SELECT  a.name, 
        b.desc, 
        IFNULL(T3.desc1, T4.desc2) as Output
FROM    table1 a 
        INNER JOIN table2 b ON a.id = b.id
        INNER JOIN 
        (
            SELECT b2.id, b2.status,  MAX(b2.created_time) as max_time
            FROM oac.qualys_scan b2
            GROUP BY b2.id  ,  b2.qualys_type
        ) t on t.id = a.id
              AND  t.status=b.status
              AND t.max_time = b.created_time
        LEFT JOIN table3 T3 on b.tid = T3.tid
        LEFT JOIN table4 T4 on b.tid = T4.tid
WHERE   b.status = 'FAIL'

【讨论】：