四、MySQL分页查询 + 子查询复习 学习笔记 （复习连接查询相关内容 详解）

8：分页查询

应用场景：当要显示的数据，一页显示不全，需要分页提交sql请求
语法：
SELECT 查询列表
FROM 表名
【JOIN type JOIN 表2
ON 连接条件
WHERE 筛选条件
GROUP BY 分组字段
HAVING 分组后的筛选
ORDER BY 排序的字段】
LIMIT 要显示条目的起始索引（起始索引从0开始），长度（要显示的条目个数）

特点：
1.LIMIT 语句放在查询语句的最后
2.上述代码的执行顺序为：
1)FROM 表名
2)JOIN type JOIN 表2
3)ON 连接条件
4)WHERE 筛选条件
5)GROUP BY 分组字段
6)HAVING 分组后的筛选
7)SELECT 查询列表
8)ORDER BY 排序的字段
9)LIMIT 要显示条目的起始索引（起始索引从0开始），长度（要显示的条目个数）

3.公式
要显示的页数page , 每页的条目数size
SELECT 查询列表
from 表名
LIMIT (page-1)*size,size;

				   page    				 第n页的起始索引
				     1							 0
				     2							10
				     3							20

# 【案例1】 查询前五条员工信息
SELECT * FROM employees LIMIT 0,5;
# 默认起始索引从0开始，可省略不写
SELECT * FROM employees LIMIT 5;

# 【案例2】 查询第11条到第25条员工信息
SELECT * FROM employees LIMIT 10,15;

# 【案例3】 有奖金的员工信息，并且工资较高的前10名显示出来
SELECT * FROM employees 
WHERE commission_pct IS NOT NULL 
ORDER BY salary DESC 
LIMIT 10;

# 【练习1】 查询所有学员的邮箱的用户名
SELECT SUBSTR(email,1,INSTR(email,'@')-1) 用户名
FROM stuinfo

# 【练习2】 查询男生和女生的个数
SELECT count(*) 个数 
FROM stuinfo
GROUP BY sex;

# 【练习3】 查询年龄 > 18岁的所有学生的姓名和年级名称
SELECT name, gradeName
FROM stuinfo s
INNER JOIN grade g
on s.gradeId = g.id
WHERE age >18;

# 【练习4】 查询哪个年级的最小年龄大于20岁
# 1) 查询每个年级的最小年龄
SELECT MIN(age),gradeid
FROM stuinfo
GROUP BY gradeId;

# 2) 1)的查询结果中大于20岁的
SELECT MIN(age),gradeid
FROM stuinfo
GROUP BY gradeId
HAVING MIN(age) > 20;

子查询复习：

#1.查询工资最低的员工信息：last_name，salary

# 1) 查询最低工资
SELECT MIN(salary)
FROM employees;

USE myemployees;
# 2) 查询last_name，salary，要求last_name = 1)中的查询结果
SELECT last_name , salary
FROM employees
WHERE salary = (
				SELECT MIN(salary)
				FROM employees
);

#2.查询平均工资最低的部门信息

# 方法一：
# 1)查询各部门的平均工资
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id;

# 2) 查询 1)结果上的最低平均工资
SELECT MIN(ag),department_id
FROM (
				SELECT AVG(salary) ag,department_id
				FROM employees
				GROUP BY department_id 
) ag_dep

# 3) 查询哪个部门的平均工资 = 2）查询出来的结果
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
											SELECT MIN(ag)
											FROM (
															SELECT AVG(salary) ag,department_id
															FROM employees
															GROUP BY department_id 
											) ag_dep
);

# 4) 查询部门编号 = 3)的查询结果中的部门编号的  部门信息
SELECT d.*
FROM departments d
WHERE d.department_id = (

												SELECT department_id
												FROM employees
												GROUP BY department_id
												HAVING AVG(salary) = (
																							SELECT MIN(ag)
																							FROM (
																											SELECT AVG(salary) ag,department_id
																											FROM employees
																											GROUP BY department_id 
																							) ag_dep
												)
);

# 方法二：
# 1)查询各部门的平均工资
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id;

# 2) 在 1) 的基础上升序排列，取第一条记录，就能得到平均工资最低的部门的编号
SELECT department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary) ASC
LIMIT 0,1;

# 3)查询部门信息
SELECT *
FROM departments
WHERE department_id = (
			SELECT department_id
			FROM employees
			GROUP BY department_id
			ORDER BY AVG(salary) ASC
			LIMIT 0,1
);

#3.查询平均工资最低的部门信息和该部门的平均工资
# 1)查询各部门的平均工资
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id;

# 2) 在 1) 的基础上升序排列，取第一条记录，就能得到平均工资最低的部门的编号
SELECT AVG(salary) , department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary) ASC
LIMIT 0,1;

# 3)查询部门信息(内连接)
SELECT d.* , ag
FROM departments d
INNER JOIN (
			SELECT AVG(salary) ag, department_id
			FROM employees
			GROUP BY department_id
			ORDER BY AVG(salary) ASC
			LIMIT 0,1
)ag_dep
ON d.department_id = ag_dep.department_id;



#4.查询平均工资最高的job信息
# 1)查询各部门的平均工资
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id;

# 2) 在 1) 的基础上降序排列，取第一条记录，就能得到平均工资最低的工种的编号
SELECT AVG(salary) , job_id
FROM employees
GROUP BY job_id
ORDER BY AVG(salary) DESC
LIMIT 0,1;

# 3) 在2)的基础上，筛选出 job_id = 2)查询结果中的job_id
SELECT *
FROM jobs
WHERE job_id = (
				SELECT job_id
				FROM employees
				GROUP BY job_id
				ORDER BY AVG(salary) DESC
				LIMIT 0,1
);

#5.查询平均工资高于公司平均工资的部门有哪些？

# 1) 查询各个部门的平均工资
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id;

# 2) 查询整个公司的平均工资
SELECT AVG(salary)
FROM employees;

# 3) 筛选 2)的结果集，满足平均工资 > 1)
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) > (
				SELECT AVG(salary)
				FROM employees
);


#6.查询出公司中所有manager的详细信息.

# 1) 查询出所有的manager_id
SELECT DISTINCT manager_id
FROM employees;
 

# 2) 查询详细信息，满足employee_id =  1)中查询的结果
# 方法一
SELECT *
FROM employees
WHERE employee_id = ANY(
				SELECT DISTINCT manager_id
				FROM employees
);

# 方法二
SELECT *
FROM employees
WHERE employee_id in(
				SELECT DISTINCT manager_id
				FROM employees
);

 
#7.查询各个部门的最高工资中最低的 那个部门的 最低工资是多少
# 1) 查询各个部门的最高工资 中，最低的那个部门的最高工资
SELECT MAX(salary),department_id
FROM employees
GROUP BY department_id
ORDER BY MAX(salary) ASC
LIMIT 0,1;

# 2) 查询 1) 结果的那个部门的最低工资
SELECT MIN(salary)
FROM employees
WHERE department_id = (
			SELECT department_id
			FROM employees
			GROUP BY department_id
			ORDER BY MAX(salary) ASC
			LIMIT 0,1
);



#8.查询平均工资最高的部门的manager的详细信息：last_name，department_id，email，salary

# 1) 查询各个部门的平均工资
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id;

# 2) 在 1)的查询结果中，查找平均工资最高的部门的department_id
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary) DESC
LIMIT 0,1;

# 3) 将employees 和 departments连接查询，筛选条件是 1）
SELECT last_name,d.department_id,email,salary
FROM employees e
INNER JOIN departments d
ON e.employee_id = d.manager_id
WHERE e.department_id = (
				SELECT department_id
				FROM employees
				GROUP BY department_id
				ORDER BY AVG(salary) DESC
				LIMIT 0,1
);

强化练习：

-- 一、查询每个专业的学生人数
SELECT majorid , COUNT(*)
FROM student
GROUP BY majorid;

-- 二、查询参加考试的学生中，每个学生的平均分、最高分
SELECT AVG(score) , MAX(score) , studentno
FROM result
GROUP BY studentno;

-- 三、查询姓张的每个学生的最低分大于60的学号、姓名
SELECT MIN(score),s.studentname,s.studentno
FROM student s
INNER JOIN result r
ON s.studentno = r.studentno
WHERE studentname LIKE'张%'
GROUP BY studentno
HAVING MIN(score) > 60;



-- 四、查询专业生日在“1988-1-1”后的学生姓名、专业名称
SELECT s.studentname , s.majorid, m.majorname,s.borndate
FROM student s
INNER JOIN major m
ON s.majorid = m.majorid
WHERE DATEDIFF(borndate,'1988-1-1')>0;


-- 五、查询每个专业的男生人数和女生人数分别是多少
# 方法一
SELECT COUNT(*) 个数, sex , majorid
FROM student
GROUP BY sex , majorid

# 方法二
SELECT majorid, 
(SELECT COUNT(*) FROM student WHERE sex='男' AND majorid=s.majorid) 男 ,
(SELECT COUNT(*) FROM student WHERE sex='女' AND majorid=s.majorid)女
FROM student s
GROUP BY majorid;

-- 六、查询专业和张翠山一样的学生的最低分

# 1) 查询张翠山的专业编号
SELECT majorid
FROM student
WHERE studentname = '张翠山';

# 2) 查询专业编号 =  1)的所有学生的编号
SELECT studentno
FROM student
WHERE majorid = (
				SELECT majorid
				FROM student
				WHERE studentname = '张翠山'
);
# 3) 查询最低分
SELECT MIN(score)
FROM result
WHERE studentno in (
					SELECT studentno
					FROM student
					WHERE majorid = (
									SELECT majorid
									FROM student
									WHERE studentname = '张翠山'
					)
);





-- 七、查询大于60分的学生的姓名、密码、专业名
SELECT s.studentname , s.studentno , r.score , loginpwd , majorname 
FROM student s
INNER JOIN result r
ON s.studentno = r.studentno
INNER JOIN major m
ON s.majorid = m.majorid
WHERE score > 60;


-- 八、按邮箱位数分组，查询每组的学生个数
SELECT COUNT(*) , LENGTH(email)
FROM student
GROUP BY LENGTH(email);

-- 九、查询学生名、专业名、分数
SELECT studentname , m.majorid , majorname , score
FROM student s
left JOIN result r
ON s.studentno = r.studentno
INNER JOIN major m
ON s.majorid = m.majorid;


-- 十、查询哪个专业没有学生，分别用左连接和右连接实现
# 左连接
SELECT m.majorid , m.majorname , s.studentno
FROM major m
LEFT JOIN student s
ON m.majorid = s.majorid
WHERE studentno IS NULL;

# 右连接
SELECT m.majorid , m.majorname , s.studentno
FROM student s
RIGHT JOIN major m
ON m.majorid = s.majorid
WHERE studentno IS NULL;



-- 十一、查询没有成绩的学生人数
SELECT COUNT(*) 无成绩学生个数
FROM student s
LEFT JOIN result r
ON s.studentno = r.studentno
WHERE r.id IS NULL;