Oracle在11.2中增强了WITH语句，使得一些树型查询不再需要CONNECT BY语句就可以完成。
看一个简单的例子：
SQL> SELECT * FROM V$VERSION;
BANNER
--------------------------------------------------------------------------------
Oracle Database11gEnterprise Edition Release11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
CORE 11.2.0.1.0 Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production
SQL> CREATE TABLE T_TREE (
2 ID NUMBER PRIMARY KEY,
3 FATHER_ID NUMBER,
4 NAME VARCHAR2(30));
表已创建。
SQL> INSERT INTO T_TREE VALUES (1, 0, 'A');
已创建1行。
SQL> INSERT INTO T_TREE VALUES (2, 1, 'BC');
已创建1行。
SQL> INSERT INTO T_TREE VALUES (3, 1, 'DE');
已创建1行。
SQL> INSERT INTO T_TREE VALUES (4, 1, 'FG');
已创建1行。
SQL> INSERT INTO T_TREE VALUES (5, 2, 'HIJ');
已创建1行。
SQL> INSERT INTO T_TREE VALUES (6, 4, 'KLM');
已创建1行。
SQL> INSERT INTO T_TREE VALUES (7, 6, 'NOPQ');
已创建1行。
SQL> INSERT INTO T_TREE VALUES (8, 5, 'RSTU');
已创建1行。
SQL> COMMIT;
提交完成。
SQL> SELECT * FROM T_TREE;
ID FATHER_ID NAME
---------- ---------- ------------------------------
1 0 A
2 1 BC
3 1 DE
4 1 FG
5 2 HIJ
6 4 KLM
7 6 NOPQ
8 5 RSTU
已选择8行。
看一个树型查询的例子：
SQL> SELECT *
2 FROM T_TREE
3 START WITH ID = 2
4 CONNECT BY PRIOR ID = FATHER_ID;
ID FATHER_ID NAME
---------- ---------- ------------------------------
2 1 BC
5 2 HIJ
8 5 RSTU
利用递归的WITH语句，可以实现同样的功能：
SQL> WITH A (ID, FATHER_ID, NAME) AS
2 (SELECT ID, FATHER_ID, NAME
3 FROM T_TREE
4 WHERE ID = 2
5 UNION ALL
6 SELECT B.ID, B.FATHER_ID, B.NAME
7 FROM A, T_TREE B
8 WHERE A.ID = B.FATHER_ID)
9 SELECT * FROM A;
ID FATHER_ID NAME
---------- ---------- ------------------------------
2 1 BC
5 2 HIJ
8 5 RSTU
这种写法与树型查询相比显得更加清晰易懂。
检查二者的执行计划：
SQL> SET AUTOT ON EXP
SQL> WITH A (ID, FATHER_ID, NAME) AS
2 (SELECT ID, FATHER_ID, NAME
3 FROM T_TREE
4 WHERE ID = 2
5 UNION ALL
6 SELECT B.ID, B.FATHER_ID, B.NAME
7 FROM A, T_TREE B
8 WHERE A.ID = B.FATHER_ID)
9 SELECT * FROM A;
ID FATHER_ID NAME
---------- ---------- ------------------------------
2 1 BC
5 2 HIJ
8 5 RSTU
执行计划
Plan hash value: 374960264
--------------------------------------------------------------------------------------------
|Id|Operation |Name |Rows|Bytes|Cost (%CPU)|Time |
--------------------------------------------------------------------------------------------
| 0|SELECT STATEMENT | | 2| 86| 7 (15)|00:00:01|
| 1| VIEW | | 2| 86| 7 (15)|00:00:01|
| 2| UNION ALL (RECURSIVE WITH) BREADTH FIRST| | | | | |
| 3| TABLE ACCESS BY INDEX ROWID |T_TREE | 1| 43| 1 (0)|00:00:01|
|*4| INDEX UNIQUE SCAN |SYS_C0011143| 1| | 1 (0)|00:00:01|
|*5| HASH JOIN | | 1| 56| 6 (17)|00:00:01|
| 6| RECURSIVE WITH PUMP | | | | | |
| 7| TABLE ACCESS FULL |T_TREE | 8| 344| 4 (0)|00:00:01|
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("ID"=2)
5 - access("A"."ID"="B"."FATHER_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
SQL> SELECT *
2 FROM T_TREE
3 START WITH ID = 2
4 CONNECT BY PRIOR ID = FATHER_ID;
ID FATHER_ID NAME
---------- ---------- ------------------------------
2 1 BC
5 2 HIJ
8 5 RSTU
执行计划Plan hash value: 856284266
-------------------------------------------------------------------------------------------
| Id | Operation |Name |Rows| Bytes |Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8| 344 | 5 (20)| 00:00:01 |
|* 1 | CONNECT BY NO FILTERING WITH START-WITH| | | | | |
| 2 | TABLE ACCESS FULL |T_TREE| 8| 344 | 4 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T_TREE"."FATHER_ID"=PRIOR "T_TREE"."ID")
filter("ID"=2)
Note
-----
- dynamic sampling used for this statement (level=2)
可以看到，虽然实现了相同的功能，但是两种方法的执行计划相去甚远，置于哪种方式效率更高，可能需要具体的测试才能确定
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