随机选择数据

[wang7890]

表A 有如下数据：

ID IDNAME TYPE
1 q 1
2 w 1
3 e 1
4 r 1
5 t 1
6 y 2
7 u 2
8 i 2
9 o 2
10 p 2
11 a 3
12 s 3
13 d 3
14 f 3
15 g 3
16 h 3

从中随机选择3个TYPE 为1 ，2个TYPE为2，3个TYPE为3 的数据，
结果类似如下所示：
2 w 1
3 e 1
4 r 1
9 o 2
7 u 2
13 d 3
14 f 3
15 g 3

请大家赐教，如何在一条SQL 语句中实现，我循环了3次分别取出来， 这样很是浪费时间！！

[hyrongg]

SQL> select * from t;

 ID IDNAME       TYPE
--- ------------ ----
  1 q               1
  2 w               1
  3 e               1
  4 r               1
  5 t               1
  6 y               2
  7 u               2
  8 i               2
  9 o               2
 10 p               2
 11 a               3
 12 s               3
 13 d               3
 14 f               3
 15 g               3
 16 h               3

16 rows selected

SQL> 
SQL> select * from (
  2    select  * from t where type=1 order by dbms_random.value
  3  ) where rownum<=3
  4  union
  5  select * from (
  6    select  * from t where type=2 order by dbms_random.value
  7  ) where rownum<=2
  8  union
  9  select * from (
 10    select  * from t where type=3 order by dbms_random.value
 11  ) where rownum<=3;

        ID IDNAME             TYPE
---------- ------------ ----------
         1 q                     1
         2 w                     1
         5 t                     1
         6 y                     2
         8 i                     2
        13 d                     3
        14 f                     3
        16 h                     3

8 rows selected

SQL> 
SQL> select * from (
  2    select  * from t where type=1 order by dbms_random.value
  3  ) where rownum<=3
  4  union
  5  select * from (
  6    select  * from t where type=2 order by dbms_random.value
  7  ) where rownum<=2
  8  union
  9  select * from (
 10    select  * from t where type=3 order by dbms_random.value
 11  ) where rownum<=3;

        ID IDNAME             TYPE
---------- ------------ ----------
         3 e                     1
         4 r                     1
         5 t                     1
         7 u                     2
        10 p                     2
        11 a                     3
        14 f                     3
        15 g                     3

8 rows selected

SQL> 

[yangzeDB]

--> --> (jake)生成测试数据
 
declare n number; 
 begin 
    select count(*) into n from user_tables  where table_name=upper('T_20081028');
     if n>0 then  
     execute immediate 'drop table T_20081028';
   end if;  
end;  
Create table T_20081028
as 
select 1 id ,'q' idname,1 type from dual union all
select 2,'w',1 from dual union all
select 3,'e',1 from dual union all
select 4,'r',1 from dual union all
select 5,'t',1 from dual union all
select 6,'y',2 from dual union all
select 7,'u',2 from dual union all
select 8,'i',2 from dual union all
select 9,'o',2 from dual union all
select 10,'p',2 from dual union all
select 11,'a',3 from dual union all
select 12,'s',3 from dual union all
select 13,'d',3 from dual union all
select 14,'f',3 from dual union all
select 15,'g',3 from dual union all
select 16,'h',3 from dual
;
Select * from T_20081028



select * from
( 
select * from T_20081028  where TYPE=1
order by dbms_random.value
)
where rownum <= 3
union all
select * from
( 
select * from T_20081028  where TYPE=2
order by dbms_random.value
)
where rownum <= 2
union all
select * from
( 
select * from T_20081028  where TYPE=3
order by dbms_random.value
)
where rownum <= 3;