MySQL 中的 UNION 关键字
时间:2021-12-04 作者:匿名
如果要从多个表中依次选择行或从单个表中选择多组行,都可以使用UNION作为单个结果集。
UNION 从 MySQL 4.0 开始可用。本节说明如何使用它。
假设您有两个表列出了潜在客户和实际客户,第三个表列出了您向其购买耗材的供应商,并且您希望通过合并所有三个表中的姓名和地址来创建一个邮件列表。UNION 提供了一种方法来做到这一点。假设三个表具有以下内容:
mysql> SELECT * FROM prospect; +---------+-------+------------------------+ | fname | lname | addr | +---------+-------+------------------------+ | Peter | Jones | 482 Rush St., Apt. 402 | | Bernice | Smith | 916 Maple Dr. | +---------+-------+------------------------+ mysql> SELECT * FROM customer; +-----------+------------+---------------------+ | last_name | first_name | address | +-----------+------------+---------------------+ | Peterson | Grace | 16055 Seminole Ave. | | Smith | Bernice | 916 Maple Dr. | | Brown | Walter | 8602 1st St. | +-----------+------------+---------------------+ mysql> SELECT * FROM vendor; +-------------------+---------------------+ | company | street | +-------------------+---------------------+ | ReddyParts, Inc. | 38 Industrial Blvd. | | Parts-to-go, Ltd. | 213B Commerce Park. | +-------------------+---------------------+
三个表是否具有不同的列名都没有关系。以下查询说明了如何一次从三个表中选择名称和地址:
mysql> SELECT fname, lname, addr FROM prospect -> UNION -> SELECT first_name, last_name, address FROM customer -> UNION -> SELECT company, '', street FROM vendor; +-------------------+----------+------------------------+ | fname | lname | addr | +-------------------+----------+------------------------+ | Peter | Jones | 482 Rush St., Apt. 402 | | Bernice | Smith | 916 Maple Dr. | | Grace | Peterson | 16055 Seminole Ave. | | Walter | Brown | 8602 1st St. | | ReddyParts, Inc. | | 38 Industrial Blvd. | | Parts-to-go, Ltd. | | 213B Commerce Park. | +-------------------+----------+------------------------+
如果要选择所有记录,包括重复项,请使用 ALL 跟随第一个 UNION 关键字:
mysql> SELECT fname, lname, addr FROM prospect -> UNION ALL -> SELECT first_name, last_name, address FROM customer -> UNION -> SELECT company, '', street FROM vendor; +-------------------+----------+------------------------+ | fname | lname | addr | +-------------------+----------+------------------------+ | Peter | Jones | 482 Rush St., Apt. 402 | | Bernice | Smith | 916 Maple Dr. | | Grace | Peterson | 16055 Seminole Ave. | | Bernice | Smith | 916 Maple Dr. | | Walter | Brown | 8602 1st St. | | ReddyParts, Inc. | | 38 Industrial Blvd. | | Parts-to-go, Ltd. | | 213B Commerce Park. | +-------------------+----------+------------------------+