如何将两个MySQL结果合并为一个？

Question

我有两个结果：

- | month      | first_time_buyers |
- | 2016-07-01 |                 2 |
- | 2016-08-01 |                 2 |

- | month      | returned_buyers |
- | 2016-08-01 |               1 |
- | 2016-09-01 |               2 |

我想将它们加入到一个结果中：

 - | date       | first_time_buyers | returned_buyers |
 - | 2016-07-01 |                 2 |               0 |
 - | 2016-08-01 |                 2 |               1 |
 - | 2016-09-01 |                 0 |               2 |

Answer 1

由于MySQL中没有FULL JOIN，因此您需要在外部查询中使用UNION ALL两个连接查询和group by。

SELECT month, 
       first_time_buyers, 
       returned_buyers 
FROM   (SELECT table1.month, 
               Ifnull(first_time_buyers, 0) AS first_time_buyers, 
               Ifnull(returned_buyers, 0)   AS returned_buyers 
        FROM   table1 
               LEFT JOIN table2 
                      ON table1.month = table2.month 
        GROUP  BY month 
        UNION ALL 
        SELECT table2.month, 
               Ifnull(first_time_buyers, 0) AS first_time_buyers, 
               Ifnull(returned_buyers, 0)   AS returned_buyers 
        FROM   table2 
               LEFT JOIN table1 
                      ON table2.month = table1.month 
        GROUP  BY month) t 
GROUP  BY month;