MySQL COUNT 函数

• 

  UNION 关键字

  MySQL COUNT函数是最简单的函数，在计算预计由SELECT语句返回的记录数时非常有用。要了解COUNT函数，请考虑一个employee_tbl表，该表具有以下记录

  mysql> 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，假设基于上表您要计算该表中的总行数，则可以按照以下步骤进行操作：

  mysql>SELECT COUNT(*) FROM employee_tbl ;
  +----------+
  | COUNT(*) |
  +----------+
  |    7     |
  +----------+
  1 row in set (0.01 sec)

  同样，如果您想计算Zara的记录数，则可以按以下步骤进行操作-

  mysql>SELECT COUNT(*) FROM employee_tbl
     -> WHERE name = "Zara";
  +----------+
  | COUNT(*) |
  +----------+
  |    2     |
  +----------+
  1 row in set (0.04 sec)

  注 –所有SQL查询都不区分大小写，因此如果在WHERE条件下提供ZARA或Zara，则不会有任何区别。