可以使用 UNION 從一些表中相繼選擇行，或從》》

MySQL 是從 4.0 版本起開始加入的UNION 這個關鍵字，下面就來介紹一下它的用法。

假設有3張表，它們分別列出了潛在顧客、實際顧客，以及你進貨的供貨商?，F在你想建立一個郵件列表，將這3張表中的名字與地址合并起來。使用 UNION 就能辦到這一點。假設這3張表內容如下：

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

3張表的列名稱不同也沒有關系。下面這個查詢展示了如何一起選擇3張表里的名字和地址。

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

如果想選擇所有記錄，包括那些重復記錄，可以使用 UNION ALL 命令。

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