w3resource_MySQL练习：Subquery

-- 要点：where里select
select first_name, last_name, salary
from employees
where salary>(
select salary from employees where last_name=‘Bull‘
)

-- 要点：直接where筛选
select first_name, last_name
from employees
where department_id=‘it‘

-- 要点：多层嵌套查询
select first_name, last_name
from employees
where manager_id in (   -- 通过employees表获得manager_id
    select employee_id FROM employees
    where department_id in (  -- 通过departments表获得在USA的department_id
        select department_id from departments 
        where location_id in (  -- 通过locations表获得在USA的location_id
            select location_id from locations 
            where country_id=‘US‘
        )
    )
)

-- 要点：在where中传入manager_id
select first_name, last_name
from employees
where employee_id in (
　　select distinct manager_id from employees
)

-- 要点：where
select first_name, last_name, salary
from employees
where salary>(
    select avg(salary) from employees
)

-- 要点：where + min()
select first_name, last_name, salary
from employees
where salary=(select min(salary) from employees)

-- 要点：where + min()
select first_name, last_name, salary
from employees
where salary>(select avg(salary) from employees)

-- 要点：where
select first_name, last_name, salary
from employees
where salary>(
    select max(salary) from employees where job_id=‘SH_CLERK‘
)
order by salary asc

-- 要点：where + not in
select first_name, last_name
from employees
where employee_id not in (
    select manager_id from employees
)

-- 1. 多表连接
select employee_id, first_name, last_name, department_name
from employees, departments
where employees.department_id=departments.department_id
-- 2. select内筛选
SELECT employee_id, first_name, last_name,
(SELECT department_name FROM departments d
WHERE e.department_id = d.department_id) department
FROM employees e ORDER BY department;

-- 要点：where
select employee_id, first_name, last_name, salary
from employees e
where salary>(
    select avg(salary) from employees e2 where e1.department_id = e2.department_id
)

-- 要点：判断奇偶数，使用%2进行取余
select *
from employees
where (employee_id%2<>0)

-- 1. 首先按salary进行降序排列取前五行，然后顺序排列取第一行
select *
from (select * from employees order by salary desc limit 5)
order by salary
limit 1
-- 2. where内做筛选
SELECT DISTINCT salary
FROM employees e1
WHERE 5 = (
    SELECT COUNT(DISTINCT salary)
    FROM employees e2
    WHERE e2.salary >= e1.salary
);

-- 1. 首先按salary进行排列取前四行，然后顺序排列取第一行
select *
from (select * from employees order by salary asc limit 4)
order by salary
limit 1
-- 2. where内做筛选
SELECT DISTINCT salary
FROM employees e1
WHERE 4 = (
    SELECT COUNT(DISTINCT salary)
    FROM employees e2
    WHERE e2.salary >= e1.salary
);

-- 要点：根据employee_id进行排序
select *
from employees
order by employee_id desc
limit 10

-- 要点：通过department_id进行判断
select department_id, department_name
from departments
where department_id not in (
select distinct department_id from employees
)

-- 要点：limit
select *
from employees
order by salary desc
limit 3

-- 要点：limit
select *
from employees
order by salary asc
limit 3

-- 要点：limit a, b 取数范围为：第[a+1, a+b]条记录
select *
from employees
order by salary
limit (n-1), 1