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  • MySQL中count()函数

    语法:SELECT COUNT(*) | COUNT(列名)  FROM 表名 [WHERE 条件语句]

    mysql> select * from student_info;
    +------+---------+
    | age  | name    |
    +------+---------+
    |   12 | Lili    |
    |   27 | Jack    |
    |   25 | Rose    |
    |   25 | Ann     |
    |   26 | Mark    |
    |   68 | Bill    |
    |   16 | James   |
    |   20 | Fred    |
    |    8 | Gary    |
    |   22 | Martin  |
    |   35 | Charles |
    |   31 | Joseph  |
    |   49 | Henry   |
    |   57 | Mike    |
    | NULL | Bob     |
    +------+---------+
    15 rows in set (0.00 sec)
    
    mysql> 

    (1)COUNT(*)

    统计表中多少条记录

    mysql> SELECT COUNT(*) FROM student_info;
    +----------+
    | COUNT(*) |
    +----------+
    |       15 |
    +----------+
    1 row in set (0.00 sec)
    
    mysql>

    (2)COUNT(列名)

    统计指定列中有多少条记录,不含值为NULL的

    mysql> SELECT COUNT(age) FROM student_info;
    +------------+
    | COUNT(age) |
    +------------+
    |         14 |
    +------------+
    1 row in set (0.00 sec)
    
    mysql>

    (3)COUNT(DISTINCT 列名)

    统计列中不重复的记录有多少条

    mysql> SELECT COUNT(DISTINCT age) FROM student_info;
    +---------------------+
    | COUNT(DISTINCT age) |
    +---------------------+
    |                  13 |
    +---------------------+
    1 row in set (0.00 sec)
    
    mysql>

    (4)与WHERE连用

    # 查找age在(20, 30)之间的记录有多少条
    mysql> SELECT COUNT(*) FROM student_info where age >=20 && age <=30;
    +----------+
    | COUNT(*) |
    +----------+
    |        6 |
    +----------+
    1 row in set (0.01 sec)
    
    mysql>


     


     

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  • 原文地址:https://www.cnblogs.com/Robotke1/p/3050406.html
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