无涯教程-MySQL UNION Keyword函数如果要从多个表中选择一个接一个的行,或者从一个表中选择多个行的集合

如果要从多个表中选择一个接一个的行,或者从一个表中选择多个行的集合作为一个输出集,则可以使用 UNION 。

从MySQL 4.0开始,UNION已可用。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,第三个表列出了从中购买供应商的供应商,并且您希望通过合并所有三个表的名称和地址来创建单个邮件列表。 UNION提供了一种执行此操作的方法。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. |
+-------------------+---------------------+

三个表是否都具有不同的列名并不重要。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,包括重复记录,请在第一个UNION关键字后加上ALL-

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.  |
+-------------------+----------+------------------------+

无涯教程-MySQL UNION Keyword函数

参考链接