emp_no	dept_no	from_date	to_date
10001	d001	1986-06-26	9999-01-01
10002	d001	1996-08-03	9999-01-01
10003	d002	1990-08-05	9999-01-01

请你查找所有员工的last_name和first_name以及对应的dept_name，也包括暂时没有分配部门的员工，以上例子输出如下:

示例1

输入:

drop table if exists  `departments` ; 
drop table if exists  `dept_emp` ; 
drop table if exists  `employees` ; 
CREATE TABLE `departments` (
`dept_no` char(4) NOT NULL,
`dept_name` varchar(40) NOT NULL,
PRIMARY KEY (`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`));
INSERT INTO departments VALUES('d001','Marketing');
INSERT INTO departments VALUES('d002','Finance');
INSERT INTO departments VALUES('d003','Human Resources');
INSERT INTO dept_emp VALUES(10001,'d001','1986-06-26','9999-01-01');
INSERT INTO dept_emp VALUES(10002,'d001','1996-08-03','9999-01-01');
INSERT INTO dept_emp VALUES(10003,'d002','1990-08-05','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|Marketing
Simmel|Bezalel|Marketing
Bamford|Parto|Finance
Koblick|Chirstian|None

原站题解

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

select a.last_name,a.first_name,c.dept_name from employees as a left join dept_emp as b
on a.emp_no = b.emp_no left join departments as c on b.dept_no = c.dept_no

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

select b.last_name,b.first_name,a.dept_name
from (select a.last_name,a.first_name,b.dept_no
      from employees as a left join dept_emp as b on a.emp_no=b.emp_no) as b
      left join departments as a
on a.dept_no=b.dept_no

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