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

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