MSSQL、MYSQL，ACCESSl,Oracle隨機讀取N條記錄方法

時間 2019-12-12

標籤 mssql mysql accessl oracle 隨機讀取記錄方法欄目 MySQL 简体版

原文原文鏈接

隨機取出n條記錄：
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
建議把MSSQL的聯機叢書中的 Transact-SQL 參考大體看一遍，不少東西就心中有數了
mysql中隨機提取數據庫記錄
-------------------------------------------------------------------------------
select * from tablename order by rand() limit 10
select * from tablename order by rand() limit 10
sqlserver中隨機提取數據庫記錄
--------------------------------------------------------------------------------
select top 10 * from tablename order by NEWID()
select top 10 * from tablename order by NEWID()
Access中隨機提取數據庫記錄
-------------------------------------------------------------------------------
SELECT top 10 * FROM tablename ORDER BY Rnd(FId)
SELECT top 10 * FROM tablename ORDER BY Rnd(FId)
FId:爲你當前表的ID字段名html

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mysql

Oracle取隨機數據

▲隨機查看前N條記錄(隨機讀取表內容)
select * from (select * from tb_phone_no order by sys_guid())
where rownum < 10;linux

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

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

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

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

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

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

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

注意每次取得的值都不一樣。

SAMPLE 是隨機抽樣，後面的數值是採樣百分比。
個人測試表A是10萬條，因此取0.01% 也就是萬分之一，這樣返回記錄數大約10條。
對於你的狀況，你能夠根據數據量來控制採樣百分比。

oracle中得到某表中的隨機數據記錄

大數據集表隨機取數據

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

sample（10）：含義爲檢索表中的10%數據

從Oracle8i開始Oracle提供採樣表掃描特性

Oracle訪問數據的基本方法有:
1.全表掃描
2.採樣表掃描

全表掃描(Full table Scan)
全表掃描返回表中全部的記錄。
執行全表掃描，Oracle讀表中的全部記錄，考查每一行是否知足WHERE條件。Oracle順序的讀分配給該表的每個數據塊，這樣全表掃描可以受益於多塊讀.
每一個數據塊Oracle只讀一次.

採樣表掃描(sample table scan)
採樣表掃描返回表中隨機採樣數據。
這種訪問方式須要在FROM語句中包含SAMPLE選項或者SAMPLE BLOCK選項.

SAMPLE選項:
當按行採樣來執行一個採樣表掃描時，Oracle從表中讀取特定百分比的記錄，並判斷是否知足WHERE子句以返回結果。

SAMPLE BLOCK選項:
使用此選項時，Oracle讀取特定百分比的BLOCK，考查結果集是否知足WHERE條件以返回知足條件的紀錄.

Sample_Percent:
Sample_Percent是一個數字，定義結果集中包含記錄佔總記錄數量的百分比。
Sample值應該在[0.000001,99.999999]之間。

1.使用SAMPLE選項

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選項

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.採樣前n條記錄的查詢

也能夠使用dbms_random包實現

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

對比一下SAMPLE選項

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>

主要注意如下幾點:

1.sample只對單表生效，不能用於錶鏈接和遠程表
2.sample會使SQL自動使用CBO

Oracle 隨機獲取N條數據

當咱們獲取數據時,可能會有這樣的需求,即每次從表中獲取數據時,是隨機獲取必定的記錄,而不是每次都獲取同樣的數據,這時咱們能夠採起Oracle內部一些函數,來達到這樣的目的.

   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;

   說明: sample(10)含義爲檢索表中的10%數據,sample值應該在[0.000001,99.999999]之間.

   其中 sys_guid() 和 dbms_random.value都是內部函數,經過這樣的方法,就能夠實現咱們的需求了.

   注:
        在使1)方法時,即便用sys_guid() 這種方法時,有時會獲取到相同的記錄,即和前一次查詢的結果集是同樣的,我查找了相關資料,有些說是和操做系統有關,在windows平臺下正常,獲取到的數據是隨機的,而在linux等平臺下始終是相同不變的數據集,有些說是由於sys_guid()函數自己的問題,即sys_guid()會在查詢上生成一個16字節的全局惟一標識符,這個標識符在絕大部分平臺上由一個宿主標識符和進程或進程的線程標識符組成,這就是說,它極可能是隨機的,可是並不表示必定是百分之百的這樣.

      因此,爲確保在不一樣的平臺每次讀取的數據都是隨機的,咱們大多采用2)和3)兩種方案,其中2)方案更經常使用.3)方案縮小了查詢的範圍,在查詢大表,且要提取數據不是很很少的狀況下,會對查詢速度上有必定的提升,

   另:在Oracle中通常獲取隨機數的方法是: