union的主要作用是將多個select的結果進行合併，每個select的語句執行結果必須得滿足如下要求才能進行union操作:

1. UNION必須由兩條或兩條以上的SELECT語句組成，語句之間用關鍵字UNION分隔（因此，如果組合4條SELECT語句，將要使用3個UNION關鍵字）;
2. UNION中的每個查詢必須包含相同的列、表示式或聚集函式（不過各個列不需要以相同的次序列出）;
3. 列資料型別必須相容：型別不必完全相同，但必須是DBMS可以隱含地轉換的型別（例如，不同的數值型別或不同的日期型別）;

union和union all的主要區別在於前者會去掉重複的記錄，後者不會，
例如:

mysql> select * from products where prod_price <=5; --結果4條
+---------+---------+---------------+------------+--------------------------------------+
| prod_id | vend_id | prod_name     | prod_price | prod_desc                            |
+---------+---------+---------------+------------+--------------------------------------+
 

| FC      |    1003 | Carrots       |       2.50 | Carrots (rabbit hunting season only) |
| FU1     |    1002 | Fuses         |       3.42 | 1 dozen, extra long                  |
| SLING   |    1003 | Sling         |       4.49 | Sling, one size fits all             |
| TNT1    |    1003 | TNT (1 stick) |       2.50 | TNT, red, single stick               |
+---------+---------+---------------+------------+--------------------------------------+
 

mysql> select * from products where vend_id = 1001 or vend_id = 1002; --結果5條
+---------+---------+--------------+------------+----------------------------------------------------------------+
| prod_id | vend_id | prod_name    | prod_price | prod_desc                                                      |
+---------+---------+--------------+------------+----------------------------------------------------------------+
| ANV01   |    1001 | .5 ton anvil |       5.99 | .5 ton anvil, black, complete with handy hook                  |
| ANV02   |    1001 | 1 ton anvil  |       9.99 | 1 ton anvil, black, complete with handy hook and carrying case |
| ANV03   |    1001 | 2 ton anvil  |      14.99 | 2 ton anvil, black, complete with handy hook and carrying case |
| FU1     |    1002 | Fuses        |       3.42 | 1 dozen, extra long                                            |
| OL1     |    1002 | Oil can      |       8.99 | Oil can, red                                                   |
+---------+---------+--------------+------------+----------------------------------------------------------------+

mysql> select * from products where prod_price <=5
    -> union
    -> select * from products where vend_id = 1001 or vend_id = 1002; --去掉重複行FU1,所有現在是8條
+---------+---------+---------------+------------+----------------------------------------------------------------+
| prod_id | vend_id | prod_name     | prod_price | prod_desc                                                      |
+---------+---------+---------------+------------+----------------------------------------------------------------+
| FC      |    1003 | Carrots       |       2.50 | Carrots (rabbit hunting season only)                           |
| FU1     |    1002 | Fuses         |       3.42 | 1 dozen, extra long                                            |
| SLING   |    1003 | Sling         |       4.49 | Sling, one size fits all                                       |
| TNT1    |    1003 | TNT (1 stick) |       2.50 | TNT, red, single stick                                         |
| ANV01   |    1001 | .5 ton anvil  |       5.99 | .5 ton anvil, black, complete with handy hook                  |
| ANV02   |    1001 | 1 ton anvil   |       9.99 | 1 ton anvil, black, complete with handy hook and carrying case |
| ANV03   |    1001 | 2 ton anvil   |      14.99 | 2 ton anvil, black, complete with handy hook and carrying case |
| OL1     |    1002 | Oil can       |       8.99 | Oil can, red                                                   |
+---------+---------+---------------+------------+----------------------------------------------------------------

mysql> select * from products where prod_price <=5
    -> union all
    -> select * from products where vend_id = 1001 or vend_id = 1002; -- 並沒有去掉重複行
+---------+---------+---------------+------------+----------------------------------------------------------------+
| prod_id | vend_id | prod_name     | prod_price | prod_desc                                                      |
+---------+---------+---------------+------------+----------------------------------------------------------------+
| FC      |    1003 | Carrots       |       2.50 | Carrots (rabbit hunting season only)                           |
| FU1     |    1002 | Fuses         |       3.42 | 1 dozen, extra long                                            |
| SLING   |    1003 | Sling         |       4.49 | Sling, one size fits all                                       |
| TNT1    |    1003 | TNT (1 stick) |       2.50 | TNT, red, single stick                                         |
| ANV01   |    1001 | .5 ton anvil  |       5.99 | .5 ton anvil, black, complete with handy hook                  |
| ANV02   |    1001 | 1 ton anvil   |       9.99 | 1 ton anvil, black, complete with handy hook and carrying case |
| ANV03   |    1001 | 2 ton anvil   |      14.99 | 2 ton anvil, black, complete with handy hook and carrying case |
| FU1     |    1002 | Fuses         |       3.42 | 1 dozen, extra long                                            |
| OL1     |    1002 | Oil can       |       8.99 | Oil can, red                                                   |
+---------+---------+---------------+------------+----------------------------------------------------------------+
9 rows in set (0.00 sec)