自连接查询

1、通过设置主外键来去除笛卡尔积

2、两张表的连接查询就一定有1个主外键关系

3、三张表的连接查询就一定有2个主外键关系

 

 

-- 自连接:自己连接自己，起别名
SELECT * FROM employee e1, employee e2;

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
);
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);

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

-- 求7369员工编号、姓名、经理编号和经理姓名
SELECT e1.empno '员工编号',e1.ename,e1.mgr '经理编号',e2.ename '经理名字'
FROM emp e1,emp e2
WHERE e1.mgr = e2.empno;

SELECT e1.empno '员工编号',e1.ename '员工姓名',e1.mgr '经理编号',e2.ename '经理姓名' 
FROM emp e1,emp e2 WHERE e1.mgr = e2.empno;--总统哥不在列