清晰讲解SQL语句中的外连接，通用于Mysql和Oracle，全是干货哦

create table DEPT
(
  DEPTNO int(2) not null,
  DNAME  VARCHAR(14),
  LOC    VARCHAR(13)
);
alter table DEPT
  add constraint PK_DEPT primary key (DEPTNO);
create table EMP
(
  EMPNO    int(4) primary key,
  ENAME    VARCHAR(10),
  JOB      VARCHAR(9),
  MGR      int(4),
  HIREDATE DATE,
  SAL      double(7,2),
  COMM     double(7,2),
  DEPTNO   int(2)
);
alter table EMP
  add constraint FK_DEPTNO foreign key (DEPTNO)
  references DEPT (DEPTNO);
create table SALGRADE
(
  GRADE int primary key,
  LOSAL double(7,2),
  HISAL double(7,2)
);
create table BONUS
(
  ENAME VARCHAR(10),
  JOB   VARCHAR(9),
  SAL   double(7,2),
  COMM  double(7,2)
);
commit;
insert into DEPT (DEPTNO, DNAME, LOC)
values (10, ‘ACCOUNTING‘, ‘NEW YORK‘);
insert into DEPT (DEPTNO, DNAME, LOC)
values (20, ‘RESEARCH‘, ‘DALLAS‘);
insert into DEPT (DEPTNO, DNAME, LOC)
values (30, ‘SALES‘, ‘CHICAGO‘);
insert into DEPT (DEPTNO, DNAME, LOC)
values (40, ‘OPERATIONS‘, ‘BOSTON‘);
commit;
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7369, ‘SMITH‘, ‘CLERK‘, 7902, ‘1980-12-17‘, 800, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7499, ‘ALLEN‘, ‘SALESMAN‘, 7698, ‘1981-02-20‘, 1600, 300, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7521, ‘WARD‘, ‘SALESMAN‘, 7698, ‘1981-02-22‘, 1250, 500, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7566, ‘JONES‘, ‘MANAGER‘, 7839, ‘1981-04-02‘, 2975, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7654, ‘MARTIN‘, ‘SALESMAN‘, 7698, ‘1981-09-28‘, 1250, 1400, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7698, ‘BLAKE‘, ‘MANAGER‘, 7839, ‘1981-05-01‘, 2850, null, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7782, ‘CLARK‘, ‘MANAGER‘, 7839, ‘1981-06-09‘, 2450, null, 10);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7788, ‘SCOTT‘, ‘ANALYST‘, 7566, ‘1987-04-19‘, 3000, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7839, ‘KING‘, ‘PRESIDENT‘, null, ‘1981-11-17‘, 5000, null, 10);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7844, ‘TURNER‘, ‘SALESMAN‘, 7698, ‘1981-09-08‘, 1500, 0, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7876, ‘ADAMS‘, ‘CLERK‘, 7788, ‘1987-05-23‘, 1100, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7900, ‘JAMES‘, ‘CLERK‘, 7698, ‘1981-12-03‘, 950, null, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7902, ‘FORD‘, ‘ANALYST‘, 7566, ‘1981-12-03‘, 3000, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7934, ‘MILLER‘, ‘CLERK‘, 7782, ‘1982-01-23‘, 1300, null, 10);
commit;
insert into SALGRADE (GRADE, LOSAL, HISAL)
values (1, 700, 1200);
insert into SALGRADE (GRADE, LOSAL, HISAL)
values (2, 1201, 1400);
insert into SALGRADE (GRADE, LOSAL, HISAL)
values (3, 1401, 2000);
insert into SALGRADE (GRADE, LOSAL, HISAL)
values (4, 2001, 3000);
insert into SALGRADE (GRADE, LOSAL, HISAL)
values (5, 3001, 9999);
commit;

-- 一、内连接
-- 交叉连接 cross JOIN
-- 笛卡尔积现象
SELECT * 
FROM emp
CROSS JOIN dept;
-- 使用using，只显示指定的同名字段
SELECT * 
FROM emp e
CROSS JOIN dept d
USING (deptno);
-- 使用on，指定不同名的字段
SELECT *
FROM emp e
CROSS JOIN dept d
ON(e.deptno = d.deptno);
-- natural join 自然连接 所有同名列只显示一次
SELECT *
FROM emp
NATURAL JOIN dept;
-- 二、外连接
-- 左外连接  emp左表，dept 右表，左表全部显示，包括没有数据的列
SELECT *
FROM emp e -- 左表
LEFT JOIN dept d -- 右表
on(e.deptno = d.deptno);
-- 右外连接  emp左表， dept右表，右表全部显示，包括没有数据的列
SELECT * 
FROM emp e  -- 左表
RIGHT JOIN dept d -- 右表
on(e.deptno = d.deptno);
-- 全外连接 ，oracle支持，mysql不支持
-- 可以通过关键字union进行解决,左表和右表的无对应数据列都会显示
SELECT *
FROM emp e
LEFT JOIN dept d
on(e.deptno = d.deptno);
UNION 
SELECT * 
FROM emp e
RIGHT JOIN dept d
on(e.deptno = d.deptno);
-- 三、三表查询
SELECT * FROM emp;
SELECT * FROM dept;
SELECT * FROM salgrade;
-- 第一步，先查两张表
SELECT e.empno,e.ename,e.sal,d.deptno,d.dname
FROM emp e
JOIN dept d
ON e.deptno = d.deptno;
-- 第二步，连接第三张表
SELECT e.empno, e.ename,e.sal,d.deptno,d.dname,sg.grade
FROM emp e
JOIN dept d
ON e.deptno = d.deptno 
JOIN salgrade sg
ON e.sal BETWEEN sg.losal AND sg.hisal; 
-- 四、自连接
-- 不是所有表都可以进行自连接，只有表中的两列有关联关系，才可以进行自连接查询
-- 将自身分解成两张表
SELECT e.empno,e.ename,e.mgr,m.empno,m.ename
FROM emp e
LEFT JOIN emp m
ON e.mgr = m.empno;