SQL

CREATE TABLE `employees` (
`emp_no` int(11) NOT NULL,
`birth_date` date NOT NULL,
`first_name` varchar(14) NOT NULL,
`last_name` varchar(16) NOT NULL,
`gender` char(1) NOT NULL,
`hire_date` date NOT NULL,
PRIMARY KEY (`emp_no`));

查找最晚入职员工的所有信息
select * from employees 
where hire_date=(select max(hire_date) from employees)

查找入职员工时间排名倒数第五的员工所有信息
SELECT * FROM employees 
where hire_date=(select distinct hire_date from employees order by hire_date desc limit 4,1)
 



多表查询:
CREATE TABLE `dept_manager` (
`dept_no` char(4) NOT NULL,
`emp_no` int(11) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`dept_no`));

CREATE TABLE `salaries` (
`emp_no` int(11) NOT NULL,
`salary` int(11) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`from_date`));

查找各个部门当前(to_date='9999-01-01')领导当前薪水详情以及其对应部门编号dept_no

select s.* ,d.dept_no from salaries as s  
join dept_manager as d  
on s.emp_no=d.emp_no 
where s.to_date = '9999-01-01' and d.to_date='9999-01-01';

SELECT s.*, d.dept_no FROM salaries s , dept_manager d 
WHERE s.to_date='9999-01-01' 
AND d.to_date='9999-01-01' 
AND s.emp_no = d.emp_no;
 



查找所有已经分配部门的员工的last_name和first_name,以及对应部门编号dept_no
CREATE TABLE `dept_emp` (
`emp_no` int(11) NOT NULL,
`dept_no` char(4) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`dept_no`));

CREATE TABLE `employees` (
`emp_no` int(11) NOT NULL,
`birth_date` date NOT NULL,
`first_name` varchar(14) NOT NULL,
`last_name` varchar(16) NOT NULL,
`gender` char(1) NOT NULL,
`hire_date` date NOT NULL,
PRIMARY KEY (`emp_no`));

select e.last_name,e.first_name,d.dept_no from employees as e
inner join dept_emp as d
where e.emp_no=d.emp_no

 
INNER JOIN 两边表同时有对应的数据，即任何一边缺失数据就不显示。
LEFT JOIN 会读取左边数据表的全部数据，即便右边表无对应数据。
RIGHT JOIN 会读取右边数据表的全部数据，即便左边表无对应数据。



查找所有员工的last_name和first_name以及对应部门编号dept_no，也包括展示没有分配具体部门的员工
CREATE TABLE `dept_emp` (
`emp_no` int(11) NOT NULL,
`dept_no` char(4) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`dept_no`));
CREATE TABLE `employees` (
`emp_no` int(11) NOT NULL,
`birth_date` date NOT NULL,
`first_name` varchar(14) NOT NULL,
`last_name` varchar(16) NOT NULL,
`gender` char(1) NOT NULL,
`hire_date` date NOT NULL,
PRIMARY KEY (`emp_no`));

SELECT ep.last_name, ep.first_name, dp.dept_no 
FROM employees ep 
LEFT JOIN dept_emp dp
ON ep.emp_no = dp.emp_no
 




查找所有员工入职时候的薪水情况，给出emp_no以及salary， 并按照emp_no进行逆序
CREATE TABLE `employees` (
`emp_no` int(11) NOT NULL,
`birth_date` date NOT NULL,
`first_name` varchar(14) NOT NULL,
`last_name` varchar(16) NOT NULL,
`gender` char(1) NOT NULL,
`hire_date` date NOT NULL,
PRIMARY KEY (`emp_no`));

CREATE TABLE `salaries` (
`emp_no` int(11) NOT NULL,
`salary` int(11) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`from_date`));

select e.emp_no, s.salary from salaries as s
inner join employees as e
on e.emp_no=s.emp_no and e.hire_date=s.from_date
order by e.emp_no desc