如何加入兩個MySQL結果轉換成一個？

Question

我有兩個結果：

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

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

我想將它們連接成一個結果，如：

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

Answer 1

由於在MySQL的沒有FULL JOIN，你需要使用UNION ALL兩個連接查詢在外部查詢中使用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;