Обсуждение: about select use random

Hi everyone

■SQL-1
CREATE TABLE wk_mclid1(
id1 integer ,
PRIMARY KEY(id1)
);

■SQL-2
INSERT INTO wk_mclid1(id1)
VALUES(1),(2),(3),(4),(5),(6),(7),(8),(9),(10)
GO

■SQL-3
select * from wk_mclid1 where id1= (round((random() * (10-1))::integer)
+ 1)::integer;



When we excute SQL-3,sometimes the result was 0 record or more than 1
record.

Why?

============================================================================================
select * from wk_mclid1 where id1= (round((random() * (10-1))::integer)
+ 1)::integer;

id1
------
9

============================================================================================
select * from wk_mclid1 where id1= (round((random() * (10-1))::integer)
+ 1)::integer;

id1
------
4
6
7
============================================================================================
select * from wk_mclid1 where id1= (round((random() * (10-1))::integer)
+ 1)::integer;

id1
------

0 record(s) selected [Fetch MetaData: 94/ms] [Fetch Data: 0/ms]
============================================================================================

thanks!

wangqi <wangqi@edgesoft.cn> writes:
> ��SQL-3
> select * from wk_mclid1 where id1= (round((random() * (10-1))::integer)
> + 1)::integer;

> When we excute SQL-3,sometimes the result was 0 record or more than 1
> record.

> Why?

random() is re-evaluated for each row scanned by the SELECT.

If you don't want that, you can use a WITH to lock down the result of a
single random() call.

            regards, tom lane

Thanks Tom, I try it.


于 2012-7-25 10:55, Tom Lane 写道:
> wangqi <wangqi@edgesoft.cn> writes:
>> ■SQL-3
>> select * from wk_mclid1 where id1= (round((random() * (10-1))::integer)
>> + 1)::integer;
>> When we excute SQL-3,sometimes the result was 0 record or more than 1
>> record.
>> Why?
> random() is re-evaluated for each row scanned by the SELECT.
>
> If you don't want that, you can use a WITH to lock down the result of a
> single random() call.
>
>             regards, tom lane
>


--
----------------------------------------------------
Yours sincerely,
Qi Wang
                         Edgesoft (China) Co.,Ltd
                         F26th Block B,Zhongli Building,
                         No.32 Huoju Road,
                         Hi-Tech Zone,Dalian,China
                         Zip:116023
                         Tel:+86-411-84753511
                         Fax:+86-411-84753577
                         http://www.edgesoft.cn
                         mailto:wangqi@edgesoft.cn
----------------------------------------------------

Hi Tom

If you don't want that, you can use a WITH to lock down the result of a
single random() call.


How should I do it?


于 2012-7-25 10:55, Tom Lane 写道:
> wangqi <wangqi@edgesoft.cn> writes:
>> ■SQL-3
>> select * from wk_mclid1 where id1= (round((random() * (10-1))::integer)
>> + 1)::integer;
>> When we excute SQL-3,sometimes the result was 0 record or more than 1
>> record.
>> Why?
> random() is re-evaluated for each row scanned by the SELECT.
>
> If you don't want that, you can use a WITH to lock down the result of a
> single random() call.
>
>             regards, tom lane
>


--
----------------------------------------------------
Yours sincerely,
Qi Wang
                         Edgesoft (China) Co.,Ltd
                         F26th Block B,Zhongli Building,
                         No.32 Huoju Road,
                         Hi-Tech Zone,Dalian,China
                         Zip:116023
                         Tel:+86-411-84753511
                         Fax:+86-411-84753577
                         http://www.edgesoft.cn
                         mailto:wangqi@edgesoft.cn
----------------------------------------------------