使用SQL UNION子句有什么要注意的-群英

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SQL UNION 子句

　SQL UNION 子句/运算符用于将两个或者更多的 SELECT 语句的运算结果组合起来。

　在使用 UNION 的时候，每个 SELECT 语句必须有相同数量的选中列、相同数量的列表达式、相同的数据类型，并且它们出现的次序要一致，不过长度不一定要相同。

语法

　UNION 子句的基本语法如下所示：

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

    UNION

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

　这里的条件可以是任何根据你的需要而设的条件。

示例

　考虑如下两张表，（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）另一张表是 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 | +-----+---------------------+-------------+--------+

　现在，让我们用 SELECT 语句将这两张表连接起来：

    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;

　结果如下所示：

    +------+----------+--------+---------------------+
    | 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 运算符用于将两个 SELECT 语句的结果组合在一起，重复行也包含在内。

　UNION ALL 运算符所遵从的规则与 UNION 一致。

语法：

　UNION ALL的基本语法如下：

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

    UNION ALL

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

示例：

　考虑如下两张表，（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）另一张表是 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 | +-----+---------------------+-------------+--------+

　现在，让我们用 SELECT 语句将这两张表连接起来：

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

　结果如下所示：

    +------+----------+--------+---------------------+
    | 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 | +------+----------+--------+---------------------+