MySQL - 合并两个表 - 有条件

【问题标题】：MySQL - merge two tables - with conditionMySQL - 合并两个表 - 有条件
【发布时间】：2016-08-22 14:30:59
【问题描述】：

我想将 2 个看起来像这样的表合并到一个表中，并为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   |     |                   |                             |
+-----------+-----------+------+-----+-------------------+-----------------------------+

【问题讨论】：

标签： mysql merge

【解决方案1】：

根据您的具体情况随意更改表名：

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，这也应该有效：

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;

【讨论】：