MySQL单表数据查询之分组数据查询之旅

简单分组查询
实现统计功能分组查询
实现多个字段分组查询

1、创建数据库

create database check_group;

2、使用刚刚创建的数据库

use check_group;

3、创建表boluo

create table boluo(     id int,        name varchar(20),      job varchar(20),       brithday date,     sal int,       depno int);

查看表的定义信息

desc boluo;

此处添加照片7

4、插入数据

insert into boluo values(001,'菠萝吹雪','洗完工',1900-01-01,1787,10), (002,'梨花诗','收银员',1900-07-07,1987,10);

insert into boluo values(001,'陆小果','洗完工',1900-02-01,1787,20), (002,'橙留香','送货员',1900-09-07,1987,20);

insert into boluo values(001,'上官子怡','洗完工',1900-02-01,1787,30), (002,'天下无贼','售货员',1900-09-07,1987,30);

查看菠萝表所有的数据记录

select * from boluo;

执行完上面的命令后，发现2出错误

1、发现出错brithday写错了，修改为birthday

alter table boluo           change brithday birthday varchar(20);

再次查看表的定义信息

+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int(11)     | YES  |              | NULL    |       |
| name     | varchar(20) | YES  |     | NULL    |       |
| job      | varchar(20) | YES  |     | NULL    |       |
| birthday | varchar(20) | YES  |     | NULL    |       |
| sal      | int(11)     | YES  |     | NULL    |       |
| depno    | int(11)     | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+

2、birthday数据没有成功插入，导致select为默认值

修改

update boluo set birthday="1900-01-01" where name='菠萝吹雪';update boluo set birthday="1900-07-07" where name='梨花诗';update boluo set birthday="1900-02-01" where name='陆小果';update boluo set birthday="1900-09-07" where name='橙留香';update boluo set birthday="1900-02-01" where name='上官子怡';update boluo set birthday="1900-09-07" where name='天下无贼';

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分组数据查询的意义：

对数据库具有基于表的特定列对数据进行分析处理

例如：可以对所有数据进行分组，然后对分组后的数据记录进行统计计算

分组用SQL语句GROUP BY实现，基本语法格式如下：

select function()  from table_name          where condition            group by field;

field字段的要求：field字段上的值一定要有重复值，否则没有意义

执行SQL语句GROUP BY对所有数据进行分组

select * from table_name        group by deptno;

实现统计功能分组查询

select group_concat(field)          from table_name                    where condition                    group by field;

==========================================

5、分组数据查询----简单分组查询

select  * from boluo group by depno;

查询结果：

+------+--------------+-----------+------------+------+-------+
| id   | name         | job            | birthday      | sal     | depno |
+------+--------------+-----------+------------+------+-------+
|    1 | 菠萝吹雪     | 洗完工    | 1900-01-01 | 1787 |    10 |
|    1 | 陆小果       | 洗完工      | 1900-02-01 | 1787 |    20 |
|    1 | 上官子怡     | 洗完工    | 1900-02-01 | 1787 |    30 |
+------+--------------+-----------+------------+------+-------+
3 rows in set (0.00 sec)

解释：根据depno字段进行分组从boluo表中查询所有的字段

只实现简单的分组查询是没有任何实际意义的，因为关键字group by单独使用时，默认查询出每个分组中随机记录一条，具有很大的不确定性。

6、分组数据查询—实现统计功能分组查询

select depno,group_concat(name) isname      from boluo     group by depno;

解释：根据depno分组，命名isname来存储显示name的字段，select 后面的depno字段，就仅仅是普通的查询而已

查询结果：

+-------+-------------------------------------------------------------------------------------------------------------------------------------------+
| depno | isname                                                                                                                                    |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------+
|    10 | 菠萝吹雪,梨花诗,菠萝吹雪,梨花诗,菠萝吹雪,梨花诗,菠萝吹雪,梨花诗,菠萝吹雪,梨花诗,菠萝吹雪,梨花诗                                           |
|    20 | 陆小果,橙留香                                                                                                                             |
|    30 | 上官子怡,天下无贼                                                                                                                         |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------+

select id,group_concat(name) isname from boluo group by depno;

查询结果：

+------+-------------------------------------------------------------------------------------------------------------------------------------------+
| id   | isname                                                                                                                                    |
+------+-------------------------------------------------------------------------------------------------------------------------------------------+
|    1 | 菠萝吹雪,梨花诗,菠萝吹雪,梨花诗,菠萝吹雪,梨花诗,菠萝吹雪,梨花诗,菠萝吹雪,梨花诗,菠萝吹雪,梨花诗                                           |
|    1 | 陆小果,橙留香                                                                                                                             |
|    1 | 上官子怡,天下无贼                                                                                                                         |
+------+-------------------------------------------------------------------------------------------------------------------------------------------+

可以比较两条语句，能看到不同点的

7、分组数据查询----实现多个字段分组查询

select depno from boluo     group by depno;

+-------+
| depno |
+-------+
|    10 |
|    20 |
|    30 |
+-------+

select depno,birthday       from boluo     group by depno,birthday;

+-------+------------+
| depno | birthday   |
+-------+------------+
|    10 | 1900-01-01 |
|    10 | 1900-07-07 |
|    20 | 1900-02-01 |
|    20 | 1900-09-07 |
|    30 | 1900-02-01 |
|    30 | 1900-09-07 |
+-------+------------+

select depno,birthday,group_concat(name),count(name)        from boluo     group by depno,birthday;

+-------+------------+-------------------------------------------------------------------------------+-------------+
| depno | birthday   | group_concat(name)                                                                          | count(name) |
+-------+------------+-------------------------------------------------------------------------------+-------------+
|    10 | 1900-01-01 | 菠萝吹雪,菠萝吹雪,菠萝吹雪,菠萝吹雪,菠萝吹雪,菠萝吹雪      |           6 |
|    10 | 1900-07-07 | 梨花诗,梨花诗,梨花诗,梨花诗,梨花诗,梨花诗                                 |           6 |
|    20 | 1900-02-01 | 陆小果                                                                                                          |           1 |
|    20 | 1900-09-07 | 橙留香                                                                                                          |           1 |
|    30 | 1900-02-01 | 上官子怡                                                                                                     |           1 |
|    30 | 1900-09-07 | 天下无贼                                                                                                     |           1 |
+-------+------------+-------------------------------------------------------------------------------+-------------+

8、分组数据查询–实现having字句限定分组查询

在MySQL软件中如果想实现对分组进行条件限制，决不能通过关键字where来实现，因为该关键字主要用来实现条件限制数据记录。为解决上述问题，MySQL软件专门提供了关键字having来实现条件限制分组数据记录。

having关键字查询语法形式如下

select function(field)      from table_name        where condition        group by field,field,field,...,fieldn      having condition;