Union in Mysql

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You can use UNION if you want to select rows one after the other from several tables or several sets of rows from a single table all as a single result set.
UNION is available as of MySQL 4.0. This section illustrates how to use it.
Suppose you have two tables that list prospective and actual customers, a third that lists vendors from whom you purchase supplies, and you want to create a single mailing list by merging names and addresses from all three tables. UNION provides a way to do this. Assume the three tables have the following contents:

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

It does not matter if all the three tables have different column names. The following query illustrates how to select names and addresses from the three tables all at once:

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

If you want to select all records, including duplicates, follow the first UNION keyword with 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.    |
+-------------------+----------+------------------------+