Join图：
Sql如下：
1）左外连接，以左边的表为主，左连接即左边的表独有+左边表和右边表共有数据
SELECT * FROM dept t
LEFT JOIN emp t2
ON t.`id_dept` = t2.`id_dept`;
2）右外连接，以右表的表为主，右连接即右边表独有+右边和左边共有数据
SELECT * FROM dept t
RIGHT JOIN emp t2
ON t.`id_dept` = t2.`id_dept`;
3）内连接，也叫自然连接同时也叫全等连接，即左边表和右边表共有数据的查询
SELECT * FROM dept t
INNER JOIN emp t2
ON t.`id_dept` = t2.`id_dept`;
4）左连接，即查询左边表的独有数据
SELECT * FROM dept t
LEFT JOIN emp t2
ON t.`id_dept` = t2.`id_dept`
WHERE t2.`id_dept` IS NULL;
5）右连接，即查询右边表独有数据
SELECT * FROM dept t
RIGHT JOIN emp t2
ON t.`id_dept` = t2.`id_dept`
WHERE t.`id_dept` IS NULL;
6）全连接，即左边表数据+右边表数据+左边表和右边表共有数据查询
SELECT * FROM dept t
LEFT JOIN emp t2
ON t.`id_dept` = t2.`id_dept`
UNION
SELECT * FROM dept t
RIGHT JOIN emp t2
ON t.`id_dept` = t2.`id_dept`;
UNION 用于合并两个或多个 SELECT 语句的结果集，并消去表中任何重复行。
UNION 内部的 SELECT 语句必须拥有相同数量的列，列也必须拥有相似的数据类型。
同时，每条 SELECT 语句中的列的顺序必须相同.7）全外连接，即左右表的共有数据之外的数据查询
SELECT * FROM dept t
FULL OUTER emp t2
ON t.`id_dept` = t2.`id_dept`
WHERE t.`id_dept` IS NULL
OR t2.`id_dept` IS NULL; --mysql不支持这种连接的语法
SELECT * FROM dept t
LEFT JOIN emp t2
ON t.`id_dept` = t2.`id_dept`
WHERE t2.`id_dept` IS NULL
UNION
SELECT * FROM dept t
RIGHT JOIN emp t2
ON t.`id_dept` = t2.`id_dept`
WHERE t.`id_dept` IS NULL;
oracle 的sql如下：
------------------自然连接 把一张表当两张表用
select e1.ename ,e2.ename from emp e1 join emp e2 on (e1.mgr = e2.empno ) ;
select ename,dname from emp join dept on (emp.deptno=dept.deptno);
---等值连接的简洁写法:
select ename ,dname from emp join dept using (deptno);
---外连接:
--左外连接
select e1.ename ,e2.ename from emp e1 left join emp e2 on (e1.mgr =
e2.empno ) ;
select e1.ename ,e2.ename from emp e1,emp e2 where e1.mgr=e2.empno(+);
--右外连接:
select e1.ename ,e2.ename from emp e1 right join emp e2 on (e1.mgr =
e2.empno ) ;
select e1.ename ,e2.ename from emp e1,emp e2 where e1.mgr(+) = e2.empno;