4 聚合 分组 排序 分页

1.聚合

• 为了快速得到统计数据，提供了5个聚合函数，
• 只能得到聚合结果，没有原始数据
• count(*)表示计算总行数，括号中写星与列名，结果是相同的

查询学生总数
select count(*) from students;

mysql> select count(*) from students where isDelete=0;
+----------+
| count(*) |
+----------+
|        6 |
+----------+

• max(列)表示求此列的最大值

查询女生的编号最大值
select max(id) from students where gender=0;

+---------+
| max(id) |
+---------+
| 6 |
+---------+

• min(列)表示求此列的最小值

查询未删除的学生最小编号
select min(id) from students where isdelete=0;

+---------+
| min(id) |
+---------+
| 1 |
+---------+

mysql> select * from students where id=1;

#子查询
mysql> select * from students where id=(select min(id) from students where isDelete=0); 
+----+--------+--------+---------------------+----------+
| id | name   | gender | birthday            | isDelete |
+----+--------+--------+---------------------+----------+
|  1 | 腾旭   |       | 1999-09-09 00:00:00 |          |
+----+--------+--------+---------------------+----------+

• sum(列)表示求此列的和

查询男生的编号之后
select sum(id) from students where gender=1;

• avg(列)表示求此列的平均值

查询未删除女生的编号平均值
select avg(id) from students where isdelete=0 and gender=0;

mysql> select avg(id) from students where gender=0 and isDelete=1;
+---------+
| avg(id) |
+---------+
|  6.0000 |
+---------+

2.分组

• 按照字段分组，表示此字段相同的数据会被放到一个组中
• 分组后，只能查询出相同的数据列，对于有差异的数据列无法出现在结果集中
• 可以对分组后的数据进行统计，做聚合运算

　　1）语法  group by

select 列1,列2,聚合... from 表名 group by 列1,列2,列3...

• 查询男女生总数

select gender as 性别,count(*)
from students
group by gender;

mysql> select count(*) from students group by gender;
+----------+
| count(*) |
+----------+
|        1 |
|        6 |
+----------+
2 rows in set (0.00 sec)

mysql> select gender,count(*) from students group by gender;
+--------+----------+
| gender | count(*) |
+--------+----------+
|        |        1 |
|       |        6 |
+--------+----------+
2 rows in set (0.01 sec)

　　2）分组后的数据筛选 having

　　对比where与having

• where是对from后面指定的表进行数据筛选，属于对原始数据的筛选
• having是对group by的结果进行筛选

　　　　

• 语法：

select 列1,列2,聚合... from 表名
group by 列1,列2,列3...
having 列1,...聚合...

• having后面的条件运算符与where的相同

查询男生总人数
方案一
select count(*)
from students
where gender=1;
-----------------------------------
方案二：
select gender as 性别,count(*)
from students
group by gender
having gender=1;

mysql> select gender,count(*) from students group by gender having gender=0;
+--------+----------+
| gender | count(*) |
+--------+----------+
|        |        1 |
+--------+----------+

mysql> select gender,count(*) from students group by gender having count(*)>2;;
+--------+----------+
| gender | count(*) |
+--------+----------+
|       |        6 |
+--------+----------+

mysql> select gender,count(*) as 人数 from students group by gender having 人数>2;
+--------+--------+
| gender | 人数   |
+--------+--------+
|       |      6 |
+--------+--------+

3.排序

• 为了方便查看数据，可以对数据进行排序
• 语法：

select * from 表名
order by 列1 asc|desc,列2 asc|desc,...

• 将行数据按照列1进行排序，如果某些行列1的值相同时，则按照列2排序，以此类推
• 默认按照列值从小到大排列
• asc从小到大排列，即升序
• desc从大到小排序，即降序

• 查询未删除男生学生信息，按学号降序

select * from students
where gender=1 and isdelete=0
order by id desc;

mysql> select * from students where isDelete=0 and gender=1 order by id desc;
+----+-----------+--------+---------------------+----------+
| id | name      | gender | birthday            | isDelete |
+----+-----------+--------+---------------------+----------+
|  8 | 腾讯云    |       | NULL                |          |
|  7 | QQ        |       | NULL                |          |
|  4 | 小米      |       | NULL                |          |
|  3 | 网易      |       | NULL                |          |
|  2 | 腾旭      |       | 1990-02-02 00:00:00 |          |
|  1 | 腾旭      |       | 1999-09-09 00:00:00 |          |
+----+-----------+--------+---------------------+----------+

• 查询未删除科目信息，按名称升序

select * from subject
where isdelete=0
order by stitle;

mysql> select * from subjects where isDelete=1 order by id asc;
+----+---------+----------+
| id | title   | isDelete |
+----+---------+----------+
|  2 | linux   |         |
|  4 | redis   |         |
|  5 | mysqlDB |         |
+----+---------+----------+

4.分页

　　1）获取部分行

• 当数据量过大时，在一页中查看数据是一件非常麻烦的事情
• 语法

select * from 表名
limit start,count

• 从start开始，获取count条数据
• start索引从0开始

+----+-----------+--------+---------------------+----------+
| id | name      | gender | birthday            | isDelete |
+----+-----------+--------+---------------------+----------+
|  1 | 腾旭      |       | 1999-09-09 00:00:00 |          |
|  2 | 腾旭      |       | 1990-02-02 00:00:00 |          |
|  3 | 网易      |       | NULL                |          |
|  4 | 小米      |       | NULL                |          |
|  6 | 酷狗      |        | 2017-02-13 00:00:00 |         |
|  7 | QQ        |       | NULL                |          |
|  8 | 腾讯云    |       | NULL                |          |
+----+-----------+--------+---------------------+----------+

mysql> select * from students limit 1,3;
+----+--------+--------+---------------------+----------+
| id | name   | gender | birthday            | isDelete |
+----+--------+--------+---------------------+----------+
|  2 | 腾旭   |       | 1990-02-02 00:00:00 |          |
|  3 | 网易   |       | NULL                |          |
|  4 | 小米   |       | NULL                |          |
+----+--------+--------+---------------------+----------+

mysql> select * from students limit 2,1;
+----+--------+--------+----------+----------+
| id | name   | gender | birthday | isDelete |
+----+--------+--------+----------+----------+
|  3 | 网易   |       | NULL     |          |
+----+--------+--------+----------+----------+

　　2）示例：分页

• 已知：每页显示m条数据，当前显示第n页
• 求总页数：此段逻辑后面会在python中实现
  • 查询总条数p1
  • 使用p1除以m得到p2
  • 如果整除则p2为总数页
  • 如果不整除则p2+1为总页数
• 求第n页的数据

select * from students
where isdelete=0
limit (n-1)*m,m

limit start,m

m=5
0,5 ==  0,1,2,3,4
1,5    ==  5,6,7,8,9
2,5 ==  10,11,12,13,14
....

m*n

m=3
0,3 == 0,1,2
1,3 == 3,4,5
2,3 == 6,7,8

n        start
1        0
2        m
(n-1)*m  

mysql> select * from students where isDelete=0 limit 0,5;
+----+--------+--------+---------------------+----------+
| id | name   | gender | birthday            | isDelete |
+----+--------+--------+---------------------+----------+
|  1 | 腾旭   |       | 1999-09-09 00:00:00 |          |
|  2 | 腾旭   |       | 1990-02-02 00:00:00 |          |
|  3 | 网易   |       | NULL                |          |
|  4 | 小米   |       | NULL                |          |
|  7 | QQ     |       | NULL                |          |
+----+--------+--------+---------------------+----------+

mysql> select * from students where isDelete=0 limit 5,5;
+----+-----------+--------+----------+----------+
| id | name      | gender | birthday | isDelete |
+----+-----------+--------+----------+----------+
|  8 | 腾讯云    |       | NULL     |          |
|  9 | 华为      |       | NULL     |          |
| 10 | 京东      |       | NULL     |          |
| 11 | 微博      |       | NULL     |          |
| 12 | 微信      |       | NULL     |          |
+----+-----------+--------+----------+----------+

总结

• 完整的select语句

select distinct *
from 表名
where ....
group by ... having ...
order by ...
limit star,count

• 执行顺序为：
  • from 表名
  • where ....
  • group by ...
  • select distinct *
  • having ...
  • order by ...
  • limit star,count
• 实际使用中，只是语句中某些部分的组合，而不是全部