半连接： 当一张表在另一张表找到匹配的记录之后，半连接（semi-jion）返回第一张表中的记录。与条件连接相反，即使在右节点中找到几条匹配的记录，左节点的表也只会返回一条记录。另外，右节点的表一条记录也不会返回。半连接通常使用IN 或 EXISTS 作为连接条件。下面是一个例子：
SQL> set linesize 999
SQL> select d.deptno,d.dname,d.loc
2 from scott.dept d
3 where d.deptno IN (select e.deptno from scott.emp e);
Execution Plan
----------------------------------------------------------
Plan hash value: 2365756639
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 69 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN SEMI | | 3 | 69 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 80 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 |
|* 4 | SORT UNIQUE | | 14 | 42 | 4 (25)| 00:00:01 |
| 5 | TABLE ACCESS FULL | EMP | 14 | 42 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("D"."DEPTNO"="E"."DEPTNO")
filter("D"."DEPTNO"="E"."DEPTNO")
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
11 consistent gets
0 physical reads
0 redo size
750 bytes sent via SQL*Net to client
492 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
3 rows processed
------------------------------------- EXISTS -----------------
SQL> select d.deptno,d.dname,d.loc
2 from scott.dept d
3 where EXISTS (select e.deptno from scott.emp e WHERE e.deptno = d.deptno);
Execution Plan
----------------------------------------------------------
Plan hash value: 2365756639
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 69 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN SEMI | | 3 | 69 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 80 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 |
|* 4 | SORT UNIQUE | | 14 | 42 | 4 (25)| 00:00:01 |
| 5 | TABLE ACCESS FULL | EMP | 14 | 42 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("E"."DEPTNO"="D"."DEPTNO")
filter("E"."DEPTNO"="D"."DEPTNO")
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
11 consistent gets
0 physical reads
0 redo size
750 bytes sent via SQL*Net to client
492 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
3 rows processed
附上半连接与条件连接的结果：
SQL> select d.deptno,d.dname,d.loc//半连接
2 from scott.dept d
3 where EXISTS (select e.deptno from scott.emp e WHERE e.deptno = d.deptno);
DEPTNO DNAME LOC
---------- -------------- -------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
SQL> select e.ename ,s.grade//条件连接
2 from emp e,salgrade s
3 where e.sal between s.losal and s.hisal;
ENAME GRADE
---------- ----------
SMITH 1
JAMES 1
ADAMS 1
WARD 2
MARTIN 2
MILLER 2
TURNER 3
ALLEN 3
CLARK 4
BLAKE 4
JONES 4
ENAME GRADE
---------- ----------
SCOTT 4
FORD 4
KING 5
14 rows selected.
可以看到 grade 有很多的重复值。
