请教一个SQL的执行顺序

Question

如题：新人请教一下SQL的执行顺序，还有一个关于Oracle中相关子查询如何理解更容易？？？
SELECT * FROM TEACHERS T1 
WHERE WAGE > (SELECT AVG(WAGE) 
FROM TEACHERS T2 
WHERE T1.DEPARTMENT_ID = T2.DEPARTMENT_ID);

Answer 1

查看下执行计划就知道了：1.explain plan for　SELECT   *   FROM   TEACHERS   T1   
WHERE   WAGE   >   (SELECT   AVG(WAGE)   
FROM   TEACHERS   T2   
WHERE   T1.DEPARTMENT_ID   =   T2.DEPARTMENT_ID);2.select * from table(DBMS_XPLAN.Display);

Answer 2

explain plan FOR SELECT   *   FROM   emp   T1   
WHERE   sal   >   (SELECT   AVG(sal)   
FROM   emp   T2   
WHERE   T1.empno   =   T2.empno);
select * from table(DBMS_XPLAN.Display);
Plan hash value: 3931844469                                                     
                                                                                
--------------------------------------------------------------------------------
| Id  | Operation            | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |         |     1 |    64 |     8  (25)| 00:00:01 |
|*  1 |  HASH JOIN           |         |     1 |    64 |     8  (25)| 00:00:01 |
|   2 |   VIEW               | VW_SQ_1 |    16 |   416 |     4  (25)| 00:00:01 |
|   3 |    SORT GROUP BY     |         |    16 |   128 |     4  (25)| 00:00:01 |
|   4 |     TABLE ACCESS FULL| EMP     |    16 |   128 |     3   (0)| 00:00:01 |
|   5 |   TABLE ACCESS FULL  | EMP     |    16 |   608 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------
                                                                                
Predicate Information (identified by operation id):                             
---------------------------------------------------                             
                                                                                
   1 - access("T1"."EMPNO"="EMPNO")                                             
       filter("SAL">"VW_COL_1")                                                 

Answer 3

1.FROM TEACHERS T1   
2.FROM TEACHERS T2   
3.WHERE T1.DEPARTMENT_ID = T2.DEPARTMENT_ID
4.SELECT AVG(WAGE)
5.SELECT   *