SQL 数据库示例：员工信息管理系统（EMP 和 DEPT 表）

创建 EMP 和 DEPT 表并插入示例数据

创建 EMP 表

CREATE TABLE emp(
	empno		INT,
	ename		VARCHAR(50),
	job		VARCHAR(50),
	mgr		INT,
	hiredate	DATE,
	sal		DECIMAL(7,2),
	comm		decimal(7,2),
	deptno		INT
);

插入 EMP 表数据

INSERT INTO emp values(7369,'SMITH','CLERK',7902,'1980-12-17',800,NULL,20);
INSERT INTO emp values(7499,'ALLEN','SALESMAN',7698,'1981-02-20',1600,300,30);
INSERT INTO emp values(7521,'WARD','SALESMAN',7698,'1981-02-22',1250,500,30);
INSERT INTO emp values(7566,'JONES','MANAGER',7839,'1981-04-02',2975,NULL,20);
INSERT INTO emp values(7654,'MARTIN','SALESMAN',7698,'1981-09-28',1250,1400,30);
INSERT INTO emp values(7698,'BLAKE','MANAGER',7839,'1981-05-01',2850,NULL,30);
INSERT INTO emp values(7782,'CLARK','MANAGER',7839,'1981-06-09',2450,NULL,10);
INSERT INTO emp values(7788,'SCOTT','ANALYST',7566,'1987-04-19',3000,NULL,20);
INSERT INTO emp values(7839,'KING','PRESIDENT',NULL,'1981-11-17',5000,NULL,10);
INSERT INTO emp values(7844,'TURNER','SALESMAN',7698,'1981-09-08',1500,0,30);
INSERT INTO emp values(7876,'ADAMS','CLERK',7788,'1987-05-23',1100,NULL,20);
INSERT INTO emp values(7900,'JAMES','CLERK',7698,'1981-12-03',950,NULL,30);
INSERT INTO emp values(7902,'FORD','ANALYST',7566,'1981-12-03',3000,NULL,20);
INSERT INTO emp values(7934,'MILLER','CLERK',7782,'1982-01-23',1300,NULL,10);

创建 DEPT 表

CREATE TABLE dept(
	deptno		INT,
	dname		varchar(14),
	loc		varchar(13)
);

插入 DEPT 表数据

INSERT INTO dept values(10, 'ACCOUNTING', 'NEW YORK');
INSERT INTO dept values(20, 'RESEARCH', 'DALLAS');
INSERT INTO dept values(30, 'SALES', 'CHICAGO');
INSERT INTO dept values(40, 'OPERATIONS', 'BOSTON');

SQL 查询示例

以下是一些常见的 SQL 查询示例，用于检索员工信息、部门信息以及员工与部门之间的关系。

1. 部门编号、部门名称、部门位置、部门人数

SELECT dept.deptno, dept.dname, dept.loc, COUNT(emp.empno) AS num_of_employees
FROM dept
LEFT JOIN emp ON dept.deptno = emp.deptno
GROUP BY dept.deptno, dept.dname, dept.loc;

2. 列出所有员工的姓名及其直接上级的姓名

SELECT e.ename, m.ename AS manager_name
FROM emp e
LEFT JOIN emp m ON e.mgr = m.empno;

3. 列出部门名称和这些部门的员工信息，同时列出那些没有员工的部门

SELECT dept.dname, emp.*
FROM dept
LEFT JOIN emp ON dept.deptno = emp.deptno
UNION
SELECT dept.dname, NULL, NULL, NULL, NULL, NULL, NULL, dept.deptno
FROM dept
LEFT JOIN emp ON dept.deptno = emp.deptno
WHERE emp.empno IS NULL;

4. 列出最低薪金大于15000的各种工作及从事此工作的员工人数

SELECT job, COUNT(*) AS num_of_employees
FROM emp
WHERE sal > 15000
GROUP BY job;

5. 列出在销售部工作的员工的姓名，假定不知道销售部的部门编号

SELECT emp.ename
FROM emp
JOIN dept ON emp.deptno = dept.deptno
WHERE dept.dname = 'SALES';

6. 列出与SMITH从事相同工作的所有员工及部门名称

SELECT emp.ename, dept.dname
FROM emp
JOIN dept ON emp.deptno = dept.deptno
WHERE emp.job = (SELECT job FROM emp WHERE ename = 'SMITH');