[每日一题] OCP1z0-047 :2013-08-08相关子查询中EXISTS的使用

正确答案是：AC

EXISTS谓词非常简单，它是对一个非空集的测试。如果在其子查询中存在任何行，则返回TRUE，否则为FALSE。该谓词不会返回UNKNOWN结果。EXIST（）谓词语法如下： <EXISTS谓词>：：＝[NOTEXISTS]<表子查询>

对exists的测试，看执行计划：

[html] 
gyj@MYDB> create table t3(id number,name varchar2(100));  
  
Table created.  
  
gyj@MYDB> insert into t3  select level,'a'||level from dual connect by level<1000000;  
  
999999 rows created.  
  
gyj@MYDB> create index idx_t3 on t3(id);  
  
Index created.  
  
gyj@MYDB> create table t4(id number,name varchar2(100));  
  
Table created.  
  
gyj@MYDB> insert into t4  select level,'a'||level from dual connect by level<10;  
  
9 rows created.  
  
gyj@MYDB> commit;  
  
Commit complete.  
  
gyj@MYDB> exec dbms_stats.gather_table_stats(user,'t3',cascade => true);  
  
PL/SQL procedure successfully completed.  
  
gyj@MYDB> exec dbms_stats.gather_table_stats(user,'t4');  
  
PL/SQL procedure successfully completed.  

看完计行计划就明白了EXISTS的执行步骤:

[html] 
gyj@MYDB> set autot traceonly;  
gyj@MYDB> select * from t4 where id in (select id from t3);  
  
9 rows selected.  
  
  
Execution Plan  
----------------------------------------------------------  
Plan hash value: 1092212754  
  
-----------------------------------------------------------------------------  
| Id  | Operation          | Name   | Rows  | Bytes | Cost (%CPU)| Time     |  
-----------------------------------------------------------------------------  
|   0 | SELECT STATEMENT   |        |     9 |    99 |    21   (0)| 00:00:01 |  
|   1 |  NESTED LOOPS SEMI |        |     9 |    99 |    21   (0)| 00:00:01 |  
|   2 |   TABLE ACCESS FULL| T4     |     9 |    54 |     3   (0)| 00:00:01 |  
|*  3 |   INDEX RANGE SCAN | IDX_T3 |   999K|  4882K|     2   (0)| 00:00:01 |  
-----------------------------------------------------------------------------  
  
Predicate Information (identified by operation id):  
---------------------------------------------------  
  
   3 - access("ID"="ID")  
  
  
Statistics  
----------------------------------------------------------  
          1  recursive calls  
          0  db block gets  
         20  consistent gets  
          0  physical reads  
          0  redo size  
        723  bytes sent via SQL*Net to client  
        520  bytes received via SQL*Net from client  
          2  SQL*Net roundtrips to/from client  
          0  sorts (memory)  
          0  sorts (disk)  
          9  rows processed

EXISTS半连接的伪代码

[html] 
open tab1  
 while tab1 still has records  
    fetch  record from tab1  
    result = false  
    open tab2  
    while tab2 still has records  
        fetch record from tab2  
        if(tab1.record matches tab2.record) then  
           result = true  
           exit loop  
         end if  
    end loop  
    close tab2  
    if (result = true) return tab1 record  
 end loop  
 close tab1