SQL COUNT() 函数 - SQL 基础教程 - 简单教程，简单编程

SQL 中的 COUNT() 函数返回匹配指定条件的行数

根据参数的不同，COUNT() 大致有三种用法

COUNT ( column_name )

COUNT(column_name) 函数返回指定列的值的数目，NULL 值除外
```
SELECT COUNT(column_name) FROM table_name;
```
COUNT(*)

COUNT(*) 函数返回表中的记录数，包括 NULL 值
```
SELECT COUNT(*) FROM table_name;
```
COUNT(DISTINCT column_name )

COUNT(DISTINCT column_name) 函数返回指定列的不同值的数目
```
SELECT COUNT(DISTINCT column_name) FROM table_name;
```
COUNT(DISTINCT) 不能在 Microsoft Access 中使用

演示数据

先在 MySQL 数据库运行下面的语句创建测试数据

CREATE DATABASE IF NOT EXISTS twle default character set utf8mb4 collate utf8mb4_unicode_ci;

USE twle;

DROP TABLE IF EXISTS `lession_views`;

CREATE TABLE `lession_views` (
    uniq bigint(20) primary key NOT NULL default '0' ,
    lession_name varchar(32) default '',
    lession_id int(11) default '0',
    date_at  int(11) NOT NULL default '0',
    views int(11) NOT NULL default '0'
);

INSERT INTO lession_views(uniq,lession_name,lession_id,date_at,views) VALUES
(20170511000001,'Python 基础教程',1,20170511,320),
(20170511000002,'Scala 基础教程', 2,20170511,22),
(20170511000003,'Ruby 基础教程', 3, 20170511,49),
(20170512000001,'Python 基础教程',1,20170512,220),
(20170512000002,'Scala 基础教程',2,20170512,12),
(20170512000003,'Ruby 基础教程',3,20170512,63),
(20170513000001,'Python 基础教程',1,20170513,441),
(20170513000002,'Scala 基础教程',2,20170513,39),
(20170513000003,'Ruby 基础教程',3,20170513,87),
(20170513000004,NULL,NULL,20170513,441);

使用 SELECT * FROM lession_views; 运行结果如下

+----------------+---------------------+------------+----------+-------+
| uniq           | lession_name        | lession_id | date_at  | views |
+----------------+---------------------+------------+----------+-------+
| 20170511000001 | Python 基础教程     |          1 | 20170511 |   320 |
| 20170511000002 | Scala 基础教程      |          2 | 20170511 |    22 |
| 20170511000003 | Ruby 基础教程       |          3 | 20170511 |    49 |
| 20170512000001 | Python 基础教程     |          1 | 20170512 |   220 |
| 20170512000002 | Scala 基础教程      |          2 | 20170512 |    12 |
| 20170512000003 | Ruby 基础教程       |          3 | 20170512 |    63 |
| 20170513000001 | Python 基础教程     |          1 | 20170513 |   441 |
| 20170513000002 | Scala 基础教程      |          2 | 20170513 |    39 |
| 20170513000003 | Ruby 基础教程       |          3 | 20170513 |    87 |
| 20170513000004 | NULL                |       NULL | 20170513 |   441 |
+----------------+---------------------+------------+----------+-------+

总共有 10 条记录

范例

我们使用 COUNT() 函数的三种形式统计一下 lession_views 表中的 lession_name 数量，顺便看看三种形式的不同之处

COUNT(lession_name)

mysql> SELECT COUNT(lession_name) FROM lession_views;
+---------------------+
| COUNT(lession_name) |
+---------------------+
|                   9 |
+---------------------+

COUNT(*)

mysql> SELECT COUNT(*) FROM lession_views;
+----------+
| COUNT(*) |
+----------+
|       10 |
+----------+

**COUNT(DISTINCT lession_name)

mysql> SELECT COUNT(DISTINCT lession_name) FROM lession_views;
+------------------------------+
| COUNT(DISTINCT lession_name) |
+------------------------------+
|                            3 |
+------------------------------+