emp_no	dept_no	from_date	to_date
10001	d001	1986-06-26	9999-01-01
10002	d002	1989-08-03	9999-01-01

示例1

输入:

drop table if exists  `dept_emp` ; 
drop table if exists  `employees` ; 
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`));
INSERT INTO dept_emp VALUES(10001,'d001','1986-06-26','9999-01-01');
INSERT INTO dept_emp VALUES(10002,'d002','1996-08-03','9999-01-01');
INSERT INTO employees VALUES(10001,'1953-09-02','Georgi','Facello','M','1986-06-26');
INSERT INTO employees VALUES(10002,'1964-06-02','Bezalel','Simmel','F','1985-11-21');
INSERT INTO employees VALUES(10003,'1959-12-03','Parto','Bamford','M','1986-08-28');
INSERT INTO employees VALUES(10004,'1954-05-01','Chirstian','Koblick','M','1986-12-01');

输出:

Facello|Georgi|d001
Simmel|Bezalel|d002
Bamford|Parto|None
Koblick|Chirstian|None

原站题解

select employees.last_name, employees.first_name, dept_emp.dept_no
from employees
left join dept_emp
on dept_emp.emp_no = employees.emp_no

select last_name,first_name,dept_no
from employees left join dept_emp on employees.emp_no = dept_emp.emp_no

select a.last_name
      ,a.first_name
      ,b.dept_no
from employees a
left outer join dept_emp b
on a.emp_no = b.emp_no

select e.last_name,e.first_name,d.dept_no from employees e left join dept_emp d on e.emp_no=d.emp_no 

select e.last_name,first_name,dept_no
from employees e left join dept_emp d on e.emp_no=d.emp_no;