SQL GROUP BY 语句

SQL 中的 GROUP BY 语句根据一个或多个列对结果集进行分组,一般配合聚合函数使用

SELECT column_name, aggregate_function(column_name) FROM table_name WHERE column_name operator value GROUP BY column_name;

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) NOT NULL default '',
    lession_id int(11) NOT NULL 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);

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 |
+----------------+---------------------+------------+----------+-------+

范例

  1. 统计各个课程的总访问量

    SELECT lession_name, SUM(views) FROM lession_views GROUP BY lession_name;
    

    运行结果输出如下

    mysql> SELECT lession_name, SUM(views) FROM lession_views GROUP BY lession_name; 
    +---------------------+------------+
    | lession_name        | SUM(views) |
    +---------------------+------------+
    | Python 基础教程      |        981 |
    | Ruby 基础教程        |        199 |
    | Scala 基础教程       |         73 |
    +---------------------+------------+
    

SQL GROUP BY 多表连接

GROUP BY 也可以用在多表连接上

我们先运行下面的语句创建一张 lession

DROP TABLE `lession`;

CREATE TABLE `lession` (
    id int(11) NOT NULL PRIMARY KEY AUTO_INCREMENT,
    name varchar(32) default '',
    views int(11) NOT NULL default '0',
    created_at DATETIME
);

INSERT INTO lession(id,name,views,created_at) VALUES
(1, 'Python 基础教程',981,'2017-04-18 13:52:03'),
(2, 'Scala 基础教程',73,'2017-04-18 16:03:32'),
(3, 'Ruby 基础教程',199,'2017-05-01 06:16:14');

使用 SELECT * FROM lession; 运行结果如下

+----+---------------------+-------+--------------------+
| id | name               | views | created_at          |
+----+---------------------+-------+--------------------+
|  1 | Python 基础教程     |   981 | 2017-04-18 13:52:03 |
|  2 | Scala 基础教程      |    73 | 2017-04-18 16:03:32 |
|  3 | Ruby 基础教程       |   199 | 2017-05-01 06:16:14 |
+----+---------------------+-------+--------------------+

总共有 3 条记录

然后我们就可以使用下面的 SQL 语句查看每门课的 总访问量

SELECT lession.name,SUM(lession_views.views) FROM lession,lession_views  WHERE lession.id=lession_views.lession_id GROUP BY lession.name;

运行结果如下

mysql> SELECT lession.name,SUM(lession_views.views) FROM lession,lession_views  WHERE lession.id=lession_views.lession_id GROUP BY lession.name;
+---------------------+--------------------------+
| name                | SUM(lession_views.views) |
+---------------------+--------------------------+
| Python 基础教程      |                      981 |
| Ruby 基础教程        |                      199 |
| Scala 基础教程       |                       73 |
+---------------------+--------------------------+

SQL 基础教程

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