union在mysql4.0以上版本才能可以使用。本节说明如何使用它。
假设有两个表，潜在和实际的客户列表，供应商购买耗材合并所有三个表中的姓名和地址，来创建一个单一的邮件列表。union提供了一种方法做到这一点。假设三个表有以下内容：
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. |+-------------------+---------------------+
这不要紧，如果所有的三个表具有不同的列名。下面的查询演示了如何选择一下子从三个表的名称和地址：
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. |+-------------------+----------+------------------------+
如果想选择所有记录，包括重复的，请all的第一个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. |+-------------------+----------+------------------------+