MSSQL、MYSQL，ACCESSl,Oracle随机读取N条记录方法

随机取出n条记录：
Sql server:select top n * from 表 order by newid()
Access:Select top n * FROM 表 orDER BY Rnd(id)
mysql:Select * From 表 order By rand() Limit n
select * from youtab where mod ( rowid , 3 ) = 0   
建议把MSSQL的联机丛书中的 Transact-SQL 参考大体看一遍，不少东西就心中有数了
mysql中随机提取数据库记录
-------------------------------------------------------------------------------
select * from tablename order by rand() limit 10
select * from tablename order by rand() limit 10
sqlserver中随机提取数据库记录
--------------------------------------------------------------------------------
select top 10 * from tablename order by NEWID()  
select top 10 * from tablename order by NEWID()
Access中随机提取数据库记录
-------------------------------------------------------------------------------
SELECT top 10 * FROM tablename ORDER BY Rnd(FId)  
SELECT top 10 * FROM tablename ORDER BY Rnd(FId)
FId:为你当前表的ID字段名

==========================================================================================

• Oracle取随机数据

▲随机查看前N条记录(随机读取表内容)
select * from (select * from tb_phone_no order by sys_guid())
where rownum < 10;

SELECT * FROM (SELECT * FROM chifan ORDER BY dbms_random.random) WHERE ROWNUM<=5

SQL> SELECT * FROM (SELECT * FROM A SAMPLE(0.01)) WHERE ROWNUM<=1;

DT
-----------------
20050105 14:59:52

SQL> SELECT * FROM (SELECT * FROM A SAMPLE(0.01)) WHERE ROWNUM<=1;

DT
-----------------
20050306 00:43:05

SQL> SELECT * FROM (SELECT * FROM A SAMPLE(0.01)) WHERE ROWNUM<=1;

DT
-----------------
20050406 02:59:50

注意每次取得的值都不一样。

SAMPLE 是随机抽样，后面的数值是采样百分比。
个人测试表A是10万条，因此取0.01% 也就是 万分之一，这样返回记录数大约10条。
对于你的状况，你能够根据数据量来控制采样百分比。

• oracle中得到某表中的随机数据记录

大数据集表随机取数据

select *
from (select *
          from table_name sample(10)
         order by trunc(dbms_random.value(0, 1000)))
where rownum = 1;

sample（10）：含义为检索表中的10%数据

从Oracle8i开始Oracle提供采样表扫描特性

Oracle访问数据的基本方法有:
1.全表扫描
2.采样表扫描

全表扫描(Full table Scan)
全表扫描返回表中全部的记录。
执行全表扫描，Oracle读表中的全部记录，考查每一行是否知足WHERE条件。Oracle顺序的读分配给该表的每个数据块，这样全表扫描可以受益于多块读.
每一个数据块Oracle只读一次.

采样表扫描(sample table scan)
采样表扫描返回表中随机采样数据。
这种访问方式须要在FROM语句中包含SAMPLE选项或者SAMPLE BLOCK选项.

SAMPLE选项:
当按行采样来执行一个采样表扫描时，Oracle从表中读取特定百分比的记录，并判断是否知足WHERE子句以返回结果。

SAMPLE BLOCK选项:
使用此选项时，Oracle读取特定百分比的BLOCK，考查结果集是否知足WHERE条件以返回知足条件的纪录.

Sample_Percent:
Sample_Percent是一个数字，定义结果集中包含记录占总记录数量的百分比。
Sample值应该在[0.000001,99.999999]之间。

1.使用SAMPLE选项

 

SQL> select * from employee SAMPLE(30); EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO ---------- ---------- --------- ---------- --------- ---------- ---------- ---------- 7369 SMITH CLERK 7902 17-DEC-80 800 20 7788 SCOTT ANALYST 7566 19-APR-87 3000 20 7839 KING PRESIDENT 17-NOV-81 5000 10 Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=25 Bytes=2175) 1 0 TABLE ACCESS (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=25 Bytes=2175) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 5 consistent gets 0 physical reads 0 redo size 880 bytes sent via SQL*Net to client 503 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 3 rows processed SQL> select * from employee SAMPLE(20); EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO ---------- ---------- --------- ---------- --------- ---------- ---------- ---------- 7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30 7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30 Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=16 Bytes=1392) 1 0 TABLE ACCESS (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=16 Bytes=1392) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 5 consistent gets 0 physical reads 0 redo size 839 bytes sent via SQL*Net to client 503 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed

 

2.使用SAMPLE BLOCK选项

 

SQL> SELECT * FROM employee SAMPLE BLOCK (50); EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO ---------- ---------- --------- ---------- --------- ---------- ---------- ---------- 7369 SMITH CLERK 7902 17-DEC-80 800 20 7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30 7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30 7566 JONES MANAGER 7839 02-APR-81 2975 20 7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30 7698 BLAKE MANAGER 7839 01-MAY-81 2850 30 7782 CLARK MANAGER 7839 09-JUN-81 2450 10 7788 SCOTT ANALYST 7566 19-APR-87 3000 20 7839 KING PRESIDENT 17-NOV-81 5000 10 7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30 10 rows selected. Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=41 Bytes=3567) 1 0 TABLE ACCESS (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=41 Bytes=3567) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 4 consistent gets 0 physical reads 0 redo size 1162 bytes sent via SQL*Net to client 503 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 10 rows processed SQL>

 

3.采样前n条记录的查询

也能够使用dbms_random包实现

 

SQL> select * from ( 2 select * from employee 3 order by dbms_random.value ) 4 where rownum <= 4; EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO ---------- ---------- --------- ---------- --------- ---------- ---------- ---------- 7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30 7839 KING PRESIDENT 17-NOV-81 5000 10 7369 SMITH CLERK 7902 17-DEC-80 800 20 7788 SCOTT ANALYST 7566 19-APR-87 3000 20 Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 1 0 COUNT (STOPKEY) 2 1 VIEW 3 2 SORT (ORDER BY STOPKEY) 4 3 TABLE ACCESS (FULL) OF 'EMPLOYEE' Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 3 consistent gets 0 physical reads 0 redo size 927 bytes sent via SQL*Net to client 503 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 4 rows processed

 

对比一下SAMPLE选项

 

SQL> SELECT * FROM employee SAMPLE (40); EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO ---------- ---------- --------- ---------- --------- ---------- ---------- ---------- 7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30 7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30 7698 BLAKE MANAGER 7839 01-MAY-81 2850 30 7839 KING PRESIDENT 17-NOV-81 5000 10 7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30 Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=33 Bytes=2871) 1 0 TABLE ACCESS (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=33 Bytes=2871) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 5 consistent gets 0 physical reads 0 redo size 961 bytes sent via SQL*Net to client 503 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 5 rows processed SQL>

 

主要注意如下几点:

1.sample只对单表生效，不能用于表链接和远程表
2.sample会使SQL自动使用CBO

• Oracle 随机获取N条数据

     当咱们获取数据时,可能会有这样的需求,即每次从表中获取数据时,是随机获取必定的记录,而不是每次都获取同样的数据,这时咱们能够采起Oracle内部一些函数,来达到这样的目的.

 

   1) select * from (select * from tablename order by sys_guid()) where rownum < N;
   2) select * from (select * from tablename order by dbms_random.value) where rownum< N;
   3) select *   from (select * from table_name sample(10)    order by trunc(dbms_random.value(0, 1000)))   where rownum < N;

 

   说明: sample(10)含义为检索表中的10%数据,sample值应该在[0.000001,99.999999]之间.

   其中 sys_guid() 和 dbms_random.value都是内部函数,经过这样的方法,就能够实现咱们的需求了.

   注:
        在使1)方法时,即便用sys_guid() 这种方法时,有时会获取到相同的记录,即和前一次查询的结果集是同样的,我查找了相关资料,有些说是和操做系统有关,在windows平台下正常,获取到的数据是随机的,而在linux等平台下始终是相同不变的数据集,有些说是由于sys_guid()函数自己的问题,即sys_guid()会在查询上生成一个16字节的全局惟一标识符,这个标识符在绝大部分平台上由一个宿主标识符和进程或进程的线程标识符组成,这就是说,它极可能是随机的,可是并不表示必定是百分之百的这样.
      
      因此,为确保在不一样的平台每次读取的数据都是随机的,咱们大多采用2)和3)两种方案,其中2)方案更经常使用.3)方案缩小了查询的范围,在查询大表,且要提取数据不是很很少的状况下,会对查询速度上有必定的提升,

   另:在Oracle中通常获取随机数的方法是:

 

select trunc(dbms_random.value(0, 1000)) from dual; (0- 1000的整数)
select dbms_random.value(0, 1000) from dual; (0-1000的浮点数)

为了您的安全，请只打开来源可靠的网址

打开网站    取消

来自: http://hi.baidu.com/siyuantlw/blog/item/4e6f03ddba1515e476c6380f.html