groupby子句的作用 GROUP BY 子句

SQL GROUP BY 子句

SQL GROUP BY 子句与 SELECT 语句结合在一起使用，可以将相同数据分成一组。

在 SELECT 语句中，GROUP BY 子句紧随 WHERE 子句，在 ORDER BY 子句之前。

语法：

GROUP BY 子句的基本语法如下所示。GROUP BY 子句必须在 WHERE 子句的条件之后，ORDER BY 子句（如果有的话）之前。

    SELECT column1, column2
    FROM table_name
    WHERE [ conditions ]
    GROUP BY column1, column2
    ORDER BY column1, column2

示例：

考虑含有如下所示记录的 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 |
    +----+----------+-----+-----------+----------+

如果你想要知道每个客户的薪水如何，可以写一个带有 GROUP BY 子句的查询：

    SQL> SELECT NAME, SUM(SALARY) FROM CUSTOMERS
         GROUP BY NAME;

结果如下所示：

    +----------+-------------+
    | NAME     | SUM(SALARY) |
    +----------+-------------+
    | Chaitali |     6500.00 |
    | Hardik   |     8500.00 |
    | kaushik  |     2000.00 |
    | Khilan   |     1500.00 |
    | Komal    |     4500.00 |
    | Muffy    |    10000.00 |
    | Ramesh   |     2000.00 |
    +----------+-------------+

现在，让我们换一张 CUSTOMERS 表，表中记录的 NAME 字段有重复值：

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

同样，如果你想要知道每个客户的薪水如何的话，可以写一个带有 GROUP BY 子句的查询：

    SQL> SELECT NAME, SUM(SALARY) FROM CUSTOMERS
         GROUP BY NAME;

结果如下所示：

    +---------+-------------+
    | NAME    | SUM(SALARY) |
    +---------+-------------+
    | Hardik  |     8500.00 |
    | kaushik |     8500.00 |
    | Komal   |     4500.00 |
    | Muffy   |    10000.00 |
    | Ramesh  |     3500.00 |
    +---------+-------------+