SQL MAX 函数

MAX函数用于找出记录集中具有最大值的记录。

要理解 MAX 函数，请考虑 employee_tbl 表，表中记录如下所示：

    SQL> SELECT * FROM employee_tbl;
    +------+------+------------+--------------------+
    | id   | name | work_date  | daily_typing_pages |
    +------+------+------------+--------------------+
    |    1 | John | 2007-01-24 |                250 |
    |    2 | Ram  | 2007-05-27 |                220 |
    |    3 | Jack | 2007-05-06 |                170 |
    |    3 | Jack | 2007-04-06 |                100 |
    |    4 | Jill | 2007-04-06 |                220 |
    |    5 | Zara | 2007-06-06 |                300 |
    |    5 | Zara | 2007-02-06 |                350 |
    +------+------+------------+--------------------+
    7 rows in set (0.00 sec)

现在，假设你想要获取 daily_typing_pages 的最大值，那么你可以用如下命令来达到目的：

    SQL> SELECT MAX(daily_typing_pages)
        -> FROM employee_tbl;
    +-------------------------+
    | MAX(daily_typing_pages) |
    +-------------------------+
    |                     350 |
    +-------------------------+
    1 row in set (0.00 sec)

你还可以使用 GROUP BY 子句，为每个名字（name）找出 daily_typing_pages 的最大值：

    SQL> SELECT id, name, MAX(daily_typing_pages)
        -> FROM employee_tbl GROUP BY name;
    +------+------+-------------------------+
    | id   | name | MAX(daily_typing_pages) |
    +------+------+-------------------------+
    |    3 | Jack |                     170 |
    |    4 | Jill |                     220 |
    |    1 | John |                     250 |
    |    2 | Ram  |                     220 |
    |    5 | Zara |                     350 |
    +------+------+-------------------------+
    5 rows in set (0.00 sec)

你还可以将 MIN 函数同 MAX 函数一起使用，来找出最小值。试一下下面的例子：

    SQL> SELECT MIN(daily_typing_pages) least, MAX(daily_typing_pages) max
        -> FROM employee_tbl;
    +-------+------+
    | least | max  |
    +-------+------+
    |   100 |  350 |
    +-------+------+
    1 row in set (0.01 sec)