COUNT() 函数

COUNT() 函数

COUNT() 函数返回匹配指定条件的行数。

COUNT(column_name) 语法

函数返回指定列的值的数目（NULL 不计入）：

SELECT COUNT(column_name) FROM table_name;

COUNT(*) 语法

COUNT(*) 函数返回表中的记录数：

SELECT COUNT(*) FROM table_name;

COUNT(DISTINCT column_name) 语法

返回指定列的不同值的数目：

SELECT COUNT(DISTINCT column_name) FROM table_name;

注释 ： COUNT(DISTINCT) 适用于 ORACLE 和 Microsoft SQL Server，但是无法用于 Microsoft Access。

示例

"access_log" ：

+-----+---------+-------+------------+
| aid | site_id | count | date       |
+-----+---------+-------+------------+
|   1 |       1 |    45 | 2016-05-10 |
|   2 |       3 |   100 | 2016-05-13 |
|   3 |       1 |   230 | 2016-05-14 |
|   4 |       2 |    10 | 2016-05-14 |
|   5 |       5 |   205 | 2016-05-14 |
|   6 |       4 |    13 | 2016-05-15 |
|   7 |       3 |   220 | 2016-05-15 |
|   8 |       5 |   545 | 2016-05-16 |
|   9 |       3 |   201 | 2016-05-17 |
+-----+---------+-------+------------+

COUNT(column_name) 实例

计算 "access_log" 表中 "site_id"=3 的总访问量：

SELECT COUNT(count) AS nums FROM access_log
WHERE site_id=3;

COUNT(*) 实例

计算 "access_log" 表中总记录数：

SELECT COUNT(*) AS nums FROM access_log;

输出结果：

mysql> SELECT COUNT(*) AS nums FROM access_log;

+-------+
|  nums |
+-------+
|  9    |
+-------+

COUNT(DISTINCT column_name) 实例

计算 "access_log" 表中不同 site_id 的记录数：

SELECT COUNT(DISTINCT site_id) AS nums FROM access_log;

输出结果：

mysql> SELECT COUNT(DISTINCT site_id) AS nums FROM access_log;

+-------+
|  nums |
+-------+
|  5    |
+-------+

练习

-- 查询所有记录的条数
select count(*) from access_log;

-- 查询 websites 表中 alexa 列中不为空的记录的条数
select count(alexa) from websites;

-- 查询 websites 表中 country 列中不重复的记录条数
select count(distinct country) from websites;