MySQL-合并两个表-有条件

Question

我想将2个这样的表合并到一个表中，并使用重复的键行在合并表中添加最旧的DateAdded值。

(Key1,Key2)是PRIMARY KEY 。

+-----------+-----------+------+-----+-------------------+-----------------------------+
| Field     | Type      | Null | Key | Default           | Extra                       |
+-----------+-----------+------+-----+-------------------+-----------------------------+
| Key1      | int(10)   | NO   | PRI |                   |                             |
| Key2      | int(10)   | NO   | PRI |                   |                             |
| DateAdded | DATETIME  | NO   |     |                   |                             |
+-----------+-----------+------+-----+-------------------+-----------------------------+

Answer 1

随意将表名更改为您的具体名称：

INSERT INTO table_merged (Key1, Key2, dateAdded)
SELECT Key1, Key2, MIN(dateAdded) dateAdded
FROM (
    SELECT Key1, Key2, dateAdded
    FROM table1
    UNION ALL
    SELECT Key1, Key2, dateAdded
    FROM table2
  ) a
GROUP BY Key1, Key2  

更新：或者，这也应该起作用：

INSERT INTO table_merged (Key1, Key2, dateAdded)
SELECT Key1, Key2, dateAdded
FROM table1
ON DUPLICATE KEY UPDATE
  dateAdded = CASE WHEN VALUES(dateAdded) < dateAdded THEN VALUES(dateAdded) ELSE dateAdded END;

INSERT INTO table_merged (Key1, Key2, dateAdded)
SELECT Key1, Key2, dateAdded
FROM table2
ON DUPLICATE KEY UPDATE
  dateAdded = CASE WHEN VALUES(dateAdded) < dateAdded THEN VALUES(dateAdded) ELSE dateAdded END;