数据库SQL实战（牛客网）：查找所有员工的last_name和first_name以及对应的dept_name

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
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));

先将employees 与dept_emp 做链接后形成新的临时表，然后再与departments 做连接形成最终的表

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;