MySQL复习值代码知识点(2)

目录

总汇表

order by使用举例

[group by使用举例](#group by使用举例)

limit举例

[group by 与order by 联合使用](#group by与order by联合使用)

having的使用

[SQL运行结果查询](# SQL运行结果查询 )

总汇表

order by	排序 desc降序 asc升序
group by	分组 （一般与聚集函数连用）
having	对组进行限制与聚集函数连用
Limit	分页
Avg	聚集函数：求平均值
Sum	聚集函数：求和（数值的加法）
Max	聚集函数：求最大值
Min	聚集函数：求最小值
count	聚集函数：相同列值求行数的和

where子句作用于表和视图，HAVING 子句作用于组；

where子句中不能使用聚集函数，而having子句中可以。

有如下表：

CREATE TABLE `grade`  (

  `id` int(11) NOT NULL PRIMARY KEY,

  `name` varchar(20),

  `grades` int(11) ,

) ;

order by使用举例

按照姓名升序排列：

mysql> select * from grade

​    -> order by name asc;

按照姓名降序排序：

mysql> select * from grade

​    -> order by name desc;

group by使用举例

group通常是和别函数一起调用的。（count,sum）

求每个学生姓名的所有成绩总分：

mysql> select name,sum(grades)

​    -> from grade

​    -> group by name;

（解释：首先，要求每个学生的总分数，要把每个学生聚集到一起，然后去把聚集在一起的每个学生的成绩求和）

求每个学生一共有几个成绩：

mysql> select name,count(name)

​    -> from grade

-> group by name;

（解释：首先把相同name的行聚集到一起，然后用count求出每个name相同的一共有几行）

求每个学生的总成绩：

mysql> select name,sum(grades)

​    -> from grade

-> group by name;

求每个学生的平均成绩：

mysql> select name,avg(grades)

​    -> from grade

​    -> group by name;

求每个学生的成绩最低分：

mysql> select name,min(grades)

​    -> from grade

​    -> group by name;

limit举例

limit 参数1：显示查询结果的前‘参数1’个数据；
limit 参数1 参数2：跳过查询结果的前‘参数1’个数据，显示剩下的前‘参数2’个数据
取前三个查询结果：

mysql> select * from grade

-> limit 3;

group by 与order by 联合使用

求总成绩排名前二的学生：

mysql> select name,sum(grades) as a

​    -> from grade

​    -> group by name

​    -> order by a desc

-> limit 2;

having的使用

查询所有学生总成绩大于100的学生：

mysql> select name,sum(grades)

​    -> from grade

​    -> group by name

​    -> having sum(grades)>100; 

SQL运行结果查询

数据输入

INSERT INTO `grade` VALUES (0, '1', 10);
INSERT INTO `grade` VALUES (1, '1', 10);
INSERT INTO `grade` VALUES (2, '2', 20);
INSERT INTO `grade` VALUES (3, '3', 30);
INSERT INTO `grade` VALUES (4, '4', 40);
INSERT INTO `grade` VALUES (5, '1', 100);
INSERT INTO `grade` VALUES (6, '2', 100);

运行结果查询：

按照姓名升序排列：
mysql>  select * from grade
    -> order by name asc;
+----+------+--------+
| id | name | grades |
+----+------+--------+
|  0 | 1    |     10 |
|  1 | 1    |     10 |
|  5 | 1    |    100 |
|  2 | 2    |     20 |
|  6 | 2    |    100 |
|  3 | 3    |     30 |
|  4 | 4    |     40 |
+----+------+--------+
按照姓名降序排序：
mysql> select * from grade
    -> order by name desc;
+----+------+--------+
| id | name | grades |
+----+------+--------+
|  4 | 4    |     40 |
|  3 | 3    |     30 |
|  2 | 2    |     20 |
|  6 | 2    |    100 |
|  0 | 1    |     10 |
|  1 | 1    |     10 |
|  5 | 1    |    100 |
+----+------+--------+
求每个学生姓名的所有成绩总分：
mysql> select name,sum(grades)
    -> from grade
    -> group by name;
+------+-------------+
| name | sum(grades) |
+------+-------------+
| 1    | 120         |
| 2    | 120         |
| 3    | 30          |
| 4    | 40          |
+------+-------------+
求每个学生一共有几个成绩：
mysql> select name,count(name)
    -> from grade
    -> group by name;
+------+-------------+
| name | count(name) |
+------+-------------+
| 1    |           3 |
| 2    |           2 |
| 3    |           1 |
| 4    |           1 |
+------+-------------+
求每个学生的总成绩：
mysql> select name,sum(grades)
    -> from grade
    ->  group by name;
+------+-------------+
| name | sum(grades) |
+------+-------------+
| 1    | 120         |
| 2    | 120         |
| 3    | 30          |
| 4    | 40          |
+------+-------------+
求每个学生的平均成绩：
mysql> select name,avg(grades)
    -> from grade
    -> group by name;
+------+-------------+
| name | avg(grades) |
+------+-------------+
| 1    | 40.0000     |
| 2    | 60.0000     |
| 3    | 30.0000     |
| 4    | 40.0000     |
+------+-------------+
求每个学生的成绩最低分：
mysql> select name,min(grades)
    -> from grade
    -> group by name;
+------+-------------+
| name | min(grades) |
+------+-------------+
| 1    |          10 |
| 2    |          20 |
| 3    |          30 |
| 4    |          40 |
+------+-------------+
取前三个查询结果：
mysql> select * from grade
    -> limit 3;
+----+------+--------+
| id | name | grades |
+----+------+--------+
|  0 | 1    |     10 |
|  1 | 1    |     10 |
|  2 | 2    |     20 |
+----+------+--------+
求总成绩排名前二的学生：
mysql> select name,sum(grades) as a
    -> from grade
    -> group by name
    -> order by a desc
    -> limit 2;
+------+-----+
| name | a   |
+------+-----+
| 1    | 120 |
| 2    | 120 |
+------+-----+
查询所有学生总成绩大于100的学生：
mysql> select name,sum(grades)
    -> from grade
    -> group by name
    -> having sum(grades)>100;
+------+-------------+
| name | sum(grades) |
+------+-------------+
| 1    | 120         |
| 2    | 120         |
+------+-------------+