inner join left right full +
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SELECT * FROM dept;
SELECT * FROM emp;
--inner join 意思是内连接 把匹配的信息全部查出来,如果关联字段中主表为空,则不显示
select e.empno,e.ename,e.job,d.deptno,d.dname
from emp e
INNER join dept d
on e.deptno=d.deptno
ORDER by e.empno;
--上面的SQL等效于
select e.empno,e.ename,e.job,d.deptno,d.dname
FROM emp e,dept d
WHERE e.deptno = d.deptno;
--left join 左连接 意思是包含左边表所有记录,右边所有的匹配的记录,如果没有则用空补齐
--解释SQL :这条SQL语句左边是EMP表 左连接结果查询出EMP所有的记录,然后根据左边表匹配出右边表DEPT所有的记录
select e.empno,e.ename,e.job,d.deptno,d.dname
from emp e
LEFT join dept d
on e.deptno=d.deptno
ORDER by e.empno;
--+在右边 为左连接
select e.empno,e.ename,e.job,d.deptno,d.dname from emp e,dept d where e.deptno=d.deptno(+) ORDER by e.empno;
--right join 右连接 意思是包括右边表所有记录,匹配左边表的记录,如果没有则以空补齐
--解释SQL:这条SQL语句EMP在左边,DEPT在右边,然后我们采用右连接,就查出右边表所有的数据
select e.empno,e.ename,e.job,d.deptno,d.dname
from emp e
RIGHT join dept d
on e.deptno=d.deptno ORDER by e.empno;
--+在左边 为右连接
select e.empno,e.ename,e.job,d.deptno,d.dname
from emp e,dept d where e.deptno(+)=d.deptno ORDER by e.empno;
--full join 全连接 意思是左右表所有的记录全部显示出来
--解释SQL:这条SQL语句采用全连接,查询出左表和右表所有的值出来
select e.empno,e.ename,e.job,d.deptno,d.dname
from emp e FULL join
dept d on e.deptno=d.deptno ORDER by e.empno;
-------------------------------
innder join查询的是匹配的数据
full join 查询的是所有表的数据
left join查询的是主表中的数据
right join 查询的是附表中的数据
2017-10-10
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