简单介绍join,outer-join,semi-join,anti-join的区别

表连接的方式如join,semi-join,outer-join,anti-join；

表连接的实现方式如nested loop,merge,hash.

本文简单的介绍表连接的方式join,semi-join,outer-join,anti-join和适用情景。

假设２个数据源(row source)．

Emp(id pk,ename,deptno)Dept(deptno pk,dname)

如下是join

select ename,dname from emp,dept where emp.deptno=dname.deptno;

２个数据源键值一一比较，返回相互匹配的记录集

for example: nested loop join

for x in ( select * from emp ) loop for y in ( select * from dept) loop if ( x.deptno == y.deptno ) OutPut_Record(x.ename,y.dname) End if end loop end loop

outer-join

select ename,dname from emp,dept where emp.deptno= dept.deptno(+);

select ename,dname from emp,dept where emp.deptno(+)=dept.deptno;

２个数据源键值一一比较，返回相互匹配的；但如果在另外一个row source没有找到匹配的也返回记录

for example: nested loop outer-join

for x in ( select * from emp ) loop find_flag=false; for y in ( select * from dept) loop if ( x.deptno == y.deptno ) OutPut_Record(x.ename,y.dname) Find_flag=true End if end loop if ( find_flag == false ) OutPut_Record(x.ename,null) End if end loop

semi-join

select dname from dept where exists( select null from emp where emp.deptno=dept.deptno)

多在子查询exists中使用，对外部row source的每个键值，查找到内部row source匹配的第一个键值后就返回，如果找到就不用再查找内部row source其他的键值了。

for example: nested loop semi-join

for x in ( select * from dept ) loop for y in ( select * from emp) loop if ( x.deptno == y.deptno ) OutPut_Record(x.dname) Break; End if end loop end loop

anti-join

select ename,deptno from emp,dept where emp.deptno!=dept.deptno

多用于!=not in等查询；如果找到满足条件（!=not in）的不返回，不满足条件（!=not in）的返回。和join相反。

　for example: nested loop anti-join

for x in ( select * from emp ) loop for y in ( select * from dept) loop if ( x.deptno != y.deptno ) OutPut_Record(x.dname,y.deptno) End if end loop end loop

More information get document

http://download-east.oracle.com/docs/cd/A81042_01/DOC/server.816/a76992/optimops.htm#39473

转自：http://blog.itpub.net/post/901/12680