jpa+mysql，union組合查詢求和結果出錯
阿新 • • 發佈：2019-02-13
UNION ALL
有兩個表，A和B。
A表資料類似
| id | a1 | a2 | a3 | a4 | a5 |
| 1 | 5 | 0.5 | a | g1 | ... |
| 2 | 10 | 1.0 | a | g2 | ... |
| 3 | 15 | 1.5 | a | g1 | ... |
B表資料類似
| id | a1 | a2 | a3 | a4 | a5 |
| 1 | 10 | 2 | b | g2 | ... |
| 2 | 20 | 4 | b | g1 | ... |
| 3 | 30 | 6 | b | g1 | ... |
結果發現，sum(x)結果是對的，但sum(y)是錯的，為什麼呢？SELECT count(*), sum(x),sum(y),z,w FROM ( (SELECT a1 as x, a2 as y, a3 as z, a4 as w FROM A) UNION ALL (SELECT b1 as x, b3 as z, b2 as y, b4 as w FROM B) )t GROUP BY w
因為按照我的思路，UNION的select結果應該是：
| x | y | z | w |
| 5 | 0.5 | a | g1 |
| 10 | 1.0 | a | g2 |
| 15 | 1.5 | a | g1 |
| 10 | 2 | b | g2 |
| 20 | 4 | b | g1 |
| 30 | 6 | b | g1 |
但實際上結果卻是：
| x | y | z | w |
| 5 | 0.5 | a | g1 |
| 10 | 1.0 | a | g2 |
| 15 | 1.5 | a | g1 |
| 10 | b | 2 | g2 |
| 20 | b | 4 | g1 |
| 30 | b | 6 | g1 |
注意：y下面的三行不是數字，而是字母。
這時來看上面被union的兩個select的欄位順序，
第一個是：a1,a2,a3,a4，
第二個是：b1,b3,b2,b4，
雖然其對應的xyzw都沒對應錯，但是：
union查詢時並不會按你為各欄位起的別名進行一一對應，而是按你的select時的欄位順序進行對應的