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 |
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