【IT168 技术文档】Oracle9i新增了WITH语法功能，可以将查询中的子查询命名，放到SELECT语句的最前面。

　　下面看一个简单的例子:

SQL> WITH
2 SEG AS (SELECT SEGMENT_NAME, SUM(BYTES)/1024 K FROM USER_SEGMENTS GROUP BY SEGMENT_NAME),
3 OBJ AS (SELECT OBJECT_NAME, OBJECT_TYPE FROM USER_OBJECTS)
4 SELECT O.OBJECT_NAME, OBJECT_TYPE, NVL(S.K, 0) SIZE_K
5 FROM OBJ O, SEG S
6 WHERE O.OBJECT_NAME = S.SEGMENT_NAME (+)
7 ;
OBJECT_NAME OBJECT_TYPE SIZE_K
DAIJC_TEST TABLE 128
P_TEST PROCEDURE 0
IND_DAIJC_TEST_C1 INDEX 128

　　通过WITH语句定义了两个子查询SEG和OBJ，在随后的SELECT语句中可以直接对预定义的子查询进行查询。从上面的例子也可以看出，使用WITH语句，将一个包含聚集、外连接等操作SQL清晰的展现出来。

　　WITH定义的子查询不仅可以使查询语句更加简单、清晰，而且WITH定义的子查询还具有在SELECT语句的任意层均可见的特点。

　　即使是在WITH的定义层中，后定义的子查询都可以使用前面已经定义好的子查询:

SQL> WITH
2 Q1 AS (SELECT 3 + 5 S FROM DUAL),
3 Q2 AS (SELECT 3 * 5 M FROM DUAL),
4 Q3 AS (SELECT S, M, S + M, S * M FROM Q1, Q2)
5 SELECT * FROM Q3;
S M S+M S*M
8 15 23 120

　　利用WITH定义查询中出现多次的子查询还能带来性能提示。Oracle会对WITH进行性能优化，当需要多次访问WITH定义的子查询时，Oracle会将子查询的结果放到一个临时表中，避免同样的子查询多次执行，从而有效的减少了查询的IO数量。

　　看一个简单的例子，首先构造一张大表，现在要取出大表中ID最小、ID最大以及ID等于平均值的记录，看看普通写法和WITH语句的区别:

SQL> CREATE TABLE T_WITH AS SELECT ROWNUM ID, A.* FROM DBA_SOURCE A WHERE ROWNUM < 100001;
表已创建。
SQL> SET TIMING ON
SQL> SET AUTOT ON
SQL> SELECT ID, NAME FROM T_WITH
2 WHERE ID IN
3 (
4 SELECT MAX(ID) FROM T_WITH
5 UNION ALL
6 SELECT MIN(ID) FROM T_WITH
7 UNION ALL
8 SELECT TRUNC(AVG(ID)) FROM T_WITH
9 );

ID NAME
1 STANDARD
50000 DBMS_BACKUP_RESTORE
100000 INITJVMAUX

已用时间: 00: 00: 00.09
执行计划
Plan hash value: 647530712

-----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |
-----------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 129 |
|* 1 | HASH JOIN | | 3 | 129 |
| 2 | VIEW | VW_NSO_1 | 3 | 39 |
| 3 | HASH UNIQUE | | 3 | 39 |
| 4 | UNION-ALL | | | |
| 5 | SORT AGGREGATE | | 1 | 13 |
| 6 | TABLE ACCESS FULL| T_WITH | 112K| 1429K|
| 7 | SORT AGGREGATE | | 1 | 13 |
| 8 | TABLE ACCESS FULL| T_WITH | 112K| 1429K|
| 9 | SORT AGGREGATE | | 1 | 13 |
| 10 | TABLE ACCESS FULL| T_WITH | 112K| 1429K|
| 11 | TABLE ACCESS FULL | T_WITH | 112K| 3299K|
-----------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - access("ID"="$nso_col_1")

Note
-----
- dynamic sampling used for this statement

统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
5529 consistent gets
0 physical reads
0 redo size
543 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
3 rows processed

　　为了避免第一次执行时物理读的影响，查询结果选取了SQL的第三次运行，物理读为0时的统计信息。