簡單的樹形查詢--prior放在select中使用

簡單的樹形查詢

使用scott用戶下的表：emp

想要查出上級主管的姓名(sql1)：

select e.empno as "員工編碼",
       e.ename as "員工姓名",
       e.mgr as "主管編碼",
       "主管姓名" as "主管姓名"
from emp e 
start with e.empno = 7566
connect by (prior e.empno) = e.mgr
order by 1;

查詢之後主管姓名要如何展示出來呢，想想可以再關聯一次emp查出姓名代碼入戲(sql2)：

select e.empno as "員工編碼",
       e.ename as "員工姓名",
       e.mgr as "主管編碼",
       b.ename  as "主管姓名"
from emp e left join emp b on e.mgr = b.empno
start with e.empno = 7566
connect by (prior e.empno) = e.mgr
order by 1;

看看執行計劃：

 Plan Hash Value  : 1231760462 
--------------------------------------------------------------------------------------------
| Id  | Operation                                  | Name | Rows | Bytes | Cost | Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                           |      |   14 |   560 |    8 | 00:00:01 |
|   1 |   SORT ORDER BY                            |      |   14 |   560 |    8 | 00:00:01 |
| * 2 |    CONNECT BY NO FILTERING WITH START-WITH |      |      |       |      |          |
| * 3 |     HASH JOIN OUTER                        |      |   14 |   336 |    7 | 00:00:01 |
|   4 |      TABLE ACCESS FULL                     | EMP  |   14 |   196 |    3 | 00:00:01 |
|   5 |      TABLE ACCESS FULL                     | EMP  |   14 |   140 |    3 | 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
------------------------------------------
* 2 - access("E"."MGR"=PRIOR "E"."EMPNO")
* 2 - filter("E"."EMPNO"=7566)
* 3 - access("E"."MGR"="B"."EMPNO"(+))

有個更簡單的方法查出主管的姓名(sql3):

select e.empno as "員工編碼",
       e.ename as "員工姓名",
       e.mgr as "主管編碼",
       prior e.ename as "主管姓名"
from emp e 
start with e.empno = 7566
connect by (prior e.empno) = e.mgr
order by 1;

查看執行計劃:

 Plan Hash Value  : 2173065311 
--------------------------------------------------------------------------------------------
| Id  | Operation                                  | Name | Rows | Bytes | Cost | Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                           |      |   14 |   462 |    4 | 00:00:01 |
|   1 |   SORT ORDER BY                            |      |   14 |   462 |    4 | 00:00:01 |
| * 2 |    CONNECT BY NO FILTERING WITH START-WITH |      |      |       |      |          |
|   3 |     TABLE ACCESS FULL                      | EMP  |   14 |   196 |    3 | 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
------------------------------------------
* 2 - access("E"."MGR"=PRIOR "E"."EMPNO")
* 2 - filter("E"."EMPNO"=7566)

是不是快了很多。

總結：之前一直認爲prior 只用在 connect by 後面，沒想到也可以用在select 裏。

如果沒有oracle環境執行sql，可以試試下面的地址：

scott學習環境