SQL UNION 子句

SQL UNION 子句/運算符用于將兩個或者更多的 SELECT 語句的運算結果組合起來。

在使用 UNION 的時候，每個 SELECT 語句必須有相同數量的選中列、相同數量的列表達式、相同的數據類型，并且它們出現的次序要一致，不過長度不一定要相同。

語法

UNION 子句的基本語法如下所示：

    SELECT column1 [, column2 ]
    FROM table1 [, table2 ]
    [WHERE condition]

    UNION

    SELECT column1 [, column2 ]
    FROM table1 [, table2 ]
    [WHERE condition]

這里的條件可以是任何根據你的需要而設的條件。

示例

考慮如下兩張表，（a）CUSTOMERS 表：

    +----+----------+-----+-----------+----------+
    | ID | NAME     | AGE | ADDRESS   | SALARY   |
    +----+----------+-----+-----------+----------+
    |  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
    |  2 | Khilan   |  25 | Delhi     |  1500.00 |
    |  3 | kaushik  |  23 | Kota      |  2000.00 |
    |  4 | Chaitali |  25 | Mumbai    |  6500.00 |
    |  5 | Hardik   |  27 | Bhopal    |  8500.00 |
    |  6 | Komal    |  22 | MP        |  4500.00 |
    |  7 | Muffy    |  24 | Indore    | 10000.00 |
    +----+----------+-----+-----------+----------+

（b）另一張表是 ORDERS 表，如下所示：

    +-----+---------------------+-------------+--------+
    |OID  | DATE                | CUSTOMER_ID | AMOUNT |
    +-----+---------------------+-------------+--------+
    | 102 | 2009-10-08 00:00:00 |           3 |   3000 |
    | 100 | 2009-10-08 00:00:00 |           3 |   1500 |
    | 101 | 2009-11-20 00:00:00 |           2 |   1560 |
    | 103 | 2008-05-20 00:00:00 |           4 |   2060 |
    +-----+---------------------+-------------+--------+

現在，讓我們用 SELECT 語句將這兩張表連接起來：

    SQL> SELECT  ID, NAME, AMOUNT, DATE
         FROM CUSTOMERS
         LEFT JOIN ORDERS
         ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
    UNION
         SELECT  ID, NAME, AMOUNT, DATE
         FROM CUSTOMERS
         RIGHT JOIN ORDERS
         ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;

結果如下所示：

    +------+----------+--------+---------------------+
    | ID   | NAME     | AMOUNT | DATE                |
    +------+----------+--------+---------------------+
    |    1 | Ramesh   |   NULL | NULL                |
    |    2 | Khilan   |   1560 | 2009-11-20 00:00:00 |
    |    3 | kaushik  |   3000 | 2009-10-08 00:00:00 |
    |    3 | kaushik  |   1500 | 2009-10-08 00:00:00 |
    |    4 | Chaitali |   2060 | 2008-05-20 00:00:00 |
    |    5 | Hardik   |   NULL | NULL                |
    |    6 | Komal    |   NULL | NULL                |
    |    7 | Muffy    |   NULL | NULL                |
    +------+----------+--------+---------------------+

UNION ALL 子句：

UNION ALL 運算符用于將兩個 SELECT 語句的結果組合在一起，重復行也包含在內。

UNION ALL 運算符所遵從的規則與 UNION 一致。

語法：

UNION ALL的基本語法如下：

    SELECT column1 [, column2 ]
    FROM table1 [, table2 ]
    [WHERE condition]

    UNION ALL

    SELECT column1 [, column2 ]
    FROM table1 [, table2 ]
    [WHERE condition]

示例：

考慮如下兩張表，（a）CUSTOMERS 表：

    +----+----------+-----+-----------+----------+
    | ID | NAME     | AGE | ADDRESS   | SALARY   |
    +----+----------+-----+-----------+----------+
    |  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
    |  2 | Khilan   |  25 | Delhi     |  1500.00 |
    |  3 | kaushik  |  23 | Kota      |  2000.00 |
    |  4 | Chaitali |  25 | Mumbai    |  6500.00 |
    |  5 | Hardik   |  27 | Bhopal    |  8500.00 |
    |  6 | Komal    |  22 | MP        |  4500.00 |
    |  7 | Muffy    |  24 | Indore    | 10000.00 |
    +----+----------+-----+-----------+----------+

（b）另一張表是 ORDERS 表，如下所示：

    +-----+---------------------+-------------+--------+
    |OID  | DATE                | CUSTOMER_ID | AMOUNT |
    +-----+---------------------+-------------+--------+
    | 102 | 2009-10-08 00:00:00 |           3 |   3000 |
    | 100 | 2009-10-08 00:00:00 |           3 |   1500 |
    | 101 | 2009-11-20 00:00:00 |           2 |   1560 |
    | 103 | 2008-05-20 00:00:00 |           4 |   2060 |
    +-----+---------------------+-------------+--------+

現在，讓我們用 SELECT 語句將這兩張表連接起來：

SQL> SELECT  ID, NAME, AMOUNT, DATE
     FROM CUSTOMERS
     LEFT JOIN ORDERS
     ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
UNION
     SELECT  ID, NAME, AMOUNT, DATE
     FROM CUSTOMERS
     RIGHT JOIN ORDERS
     ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;

結果如下所示：

    +------+----------+--------+---------------------+
    | ID   | NAME     | AMOUNT | DATE                |
    +------+----------+--------+---------------------+
    |    1 | Ramesh   |   NULL | NULL                |
    |    2 | Khilan   |   1560 | 2009-11-20 00:00:00 |
    |    3 | kaushik  |   3000 | 2009-10-08 00:00:00 |
    |    3 | kaushik  |   1500 | 2009-10-08 00:00:00 |
    |    4 | Chaitali |   2060 | 2008-05-20 00:00:00 |
    |    5 | Hardik   |   NULL | NULL                |
    |    6 | Komal    |   NULL | NULL                |
    |    7 | Muffy    |   NULL | NULL                |
    |    3 | kaushik  |   3000 | 2009-10-08 00:00:00 |
    |    3 | kaushik  |   1500 | 2009-10-08 00:00:00 |
    |    2 | Khilan   |   1560 | 2009-11-20 00:00:00 |
    |    4 | Chaitali |   2060 | 2008-05-20 00:00:00 |
    +------+----------+--------+---------------------+

另外，還有兩個子句（亦即運算法）與 UNION 子句非常相像：

• SQL INTERSECT 子句：用于組合兩個 SELECT 語句，但是只返回兩個 SELECT 語句的結果中都有的行。
• SQL EXCEPT 子句：組合兩個 SELECT 語句，并將第一個 SELECT 語句的結果中存在，但是第二個 SELECT 語句的結果中不存在的行返回。