SQL - RIGHT JOINS

  • 简述

    SQLRIGHT JOIN返回右表中的所有行,即使左表中没有匹配项。这意味着如果 ON 子句匹配左表中的 0(零)条记录;连接仍将在结果中返回一行,但左表中的每一列都为 NULL。
    这意味着右连接返回右表中的所有值,加上左表中的匹配值,如果没有匹配的连接谓词,则返回 NULL。
  • 句法

    a 的基本语法RIGHT JOIN如下。
    
    SELECT table1.column1, table2.column2...
    FROM table1
    RIGHT JOIN table2
    ON table1.common_field = table2.common_field;
    
  • 例子

    考虑以下两个表,
    Table 1− CUSTOMERS 表如下。
    
    +----+----------+-----+-----------+----------+
    | 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 |
    +----+----------+-----+-----------+----------+
    
    Table 2− ORDERS 表如下。
    
    +-----+---------------------+-------------+--------+
    |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,让我们使用 RIGHT JOIN 连接这两个表,如下所示。
    
    SQL> SELECT  ID, NAME, AMOUNT, DATE
       FROM CUSTOMERS
       RIGHT JOIN ORDERS
       ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;
    
    这将产生以下结果 -
    
    +------+----------+--------+---------------------+
    | ID   | NAME     | AMOUNT | DATE                |
    +------+----------+--------+---------------------+
    |    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 |
    +------+----------+--------+---------------------+