【牛客SQL】SQL19 查找所有员工的last_name和first_name以及对应的dept_name

题目描述

描述
有一个员工表employees简况如下:

有一个部门表departments表简况如下:

有一个，部门员工关系表dept_emp简况如下:

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');
复制
输出：
Facello|Georgi|Marketing
Simmel|Bezalel|Marketing
Bamford|Parto|Finance
Koblick|Chirstian|None

题解

运行时间：12ms
超过86.54% 用Sqlite提交的代码
占用内存：3584KB
超过8.06%用Sqlite提交的代码

SELECT e.last_name, e.first_name, t.dept_name 
FROM employees AS e LEFT JOIN (
    SELECT d1.emp_no, d1.dept_no, d2.dept_name 
    FROM dept_emp d1 LEFT JOIN departments d2
    WHERE d1.dept_no = d2.dept_no
) AS t
ON e.emp_no = t.emp_no
;