MySQL进阶8 分页查询(limit) - 【SQL查询语法执行顺序及大致结构】- 子查询的3个经典案例...

#进阶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          #7
    from  表1   #执行顺序:#1
    【join type join 表2   #2
     on 连接条件           #3
     where 筛选条件         #4
     group by 分组字段      #5
     having 分组后筛选或函数   #6
     order by 排序的字段        #8
    】
    limit offset, size ;            #9  
        #offset 要显示条目的起始索引(从0开始,0想可以省略),size 要显示的条目个数
    特点:
    ①limit语句放在查询语句的,执行上语法上都在最后
    ②公式, 要显示的页数page ,每页的条目数 size
        select 查询列表
        from 表
        limit(page-1)*size ,size;
*/
#案例1: 查询前五条员工信息
SELECT * FROM  employees LIMIT 0,5;
SELECT * FROM  employees LIMIT 5;

#案例2:查询第11到第25条
SELECT * FROM employees LIMIT 11,15;

# 案例3:有奖金的员工信息,并且工资较高的前10项显示出来
SELECT *
FROM employees
WHERE commission_pct IS NOT NULL
ORDER BY salary DESC
LIMIT 10;
#--------------------------
    #课堂练习
#1.查询所有学员的邮箱的用户名(注: 邮箱中 @前的字符) ;instr() : 返回子串的索引下标
SELECT SUBSTR( email,1,INSTR(email,'@')-1)
FROM stuinfo;
#------------------------------------
    #子查询的经典案例
    
#1:查询工资最低的员工信息: last_name,salary
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
    #方式1:
    #分组查询平均工资
    #查询最低的一组
    #查询那个部门最低平均工资 = 最低的一组
SELECT AVG(salary) ag,department_id
    FROM employees
    GROUP BY department_id;
    
SELECT MIN(ag)
FROM (
    SELECT AVG(salary) ag,department_id
    FROM employees
    GROUP BY department_id
)ag_dep;

SELECT *,AVG(salary)
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
);
    #方式2:
    #①分组查询平均工资
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id; 
    #②求出最低的(limit=1)平均工资的部门编号
SELECT department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary)
LIMIT 1;
    #③ 进行嵌套,输出该部门信息
SELECT *
FROM departments
WHERE department_id =(
    SELECT department_id
    FROM employees
    GROUP BY department_id
    ORDER BY AVG(salary)
    LIMIT 1
);

#3: 查询平均工资高于公司的平均工资的部门有哪些

#①查询平均工资
SELECT AVG(salary)
FROM employees;

#② 查询每个部门的
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id; 

#③ 进行筛选2的结果集: > ① 即可!

SELECT  *,AVG(salary) ag,department_id
FROM employees
GROUP BY department_id
HAVING ag >(
    SELECT AVG(salary)
    FROM employees
);