Question

我有三个表：orders，purchase_order和sale_order。表顺序有两列：外键order_id和order_numb。 Order_id是purchase_order或sale_order的id，order_numb只是说明它的顺序。

order_numb = 1它是purchase_order order_numb = 2是sale_order

所以我需要从表顺序中进行选择，并且对于每一行检查它是否具有order_numb = 1或order_numb = 2，并且基于该结果，根据他们的id从buy_order或sale_order中选择。

我可以使用两个查询，但我想在一个查询中完成。

ORDER

| order_id | action_numb |
--------------------------
|    51    |      1      |
--------------------------
|    40    |      2      |
--------------------------
|    41    |      2      |
--------------------------
|    52    |      1      |


PURCHASE_ORDER      

| id | other_columns |
----------------------
| 51 |   something   |
----------------------
| 52 |   something   |


SALE_ORDER      

| id | other_columns |
----------------------
| 40 |   something   |
----------------------
| 41 |   something   |
----------------------

Answer 1

SELECT o.*
     , CASE
         WHEN so.id is null THEN po.other_columns
         WHEN po.id is null THEN so.other_columns
       END other_columns
  FROM orders o
    LEFT JOIN purchase_order po
      ON    po.id = o.order_id
        AND o.order_numb = 1 
    LEFT JOIN sale_order so
      ON    so.id = o.order_id
        AND o.order_numb = 2

Answer 2

假设您从两个表中选择相同的列，您可以使用UNION来完成任务。

SELECT o.id,<other-columns>
    FROM order o INNER JOIN purchase_order po 
    ON o.id = po.id 
    WHERE o.action_numb = 1 

    UNION

    SELECT o.id,<other-columns>
    FROM order o INNER JOIN sale_order so 
    ON o.id = so.id 
    WHERE o.action_numb = 2

如何根据另一个SELECT的结果进行SELECT查询？

2 个答案: