Oracle9i数据库WITH查询语法小议

文章作者 100test 发表时间 2007:03:14 13:50:59
来源 100Test.Com百考试题网

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。