如何在表列中定义的表上LEFT JOIN

Question

考虑以下表格：

用户表

id | name | createdAt  |
-----------------------|
1  | John | 2018-02-02 |

活动表

id | itemId | itemTable | createdAt  |
-------------------------------------|
13 | 1      | User      | 2018-02-02 |
14 | 142    | Client    | 2018-02-02 |

我希望能够在列中指定的表上LEFT JOIN：

SELECT    b.* 
FROM      activity      AS a 
LEFT JOIN *a.tablename* AS b 
ON        b.id = a.itemid

用于MemSQL或MySQL

Answer 1

您无法在SELECT查询中执行此操作。 SQL不会以这种方式运行。

如果你知道只有两个表，你可以像这样表达查询：

SELECT u.* 
FROM USER u 
WHERE u.id = (SELECT 1 FROM activity a WHERE  a.itemid = u.id) 
UNION ALL 
SELECT c.* 
FROM client c 
WHERE c.id = (SELECT 1 FROM activity a WHERE  a.itemid = c.id);