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MSSQL  random select

Posted on 2013-07-01
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Last Modified: 2013-07-04
Hi,
Lets say I have a database table with 100 rows as game questions, how to select randomly 10 questions among the 100 and this random set changes every time I play again?
tks
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Question by:PM150
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Expert Comment

by:EvilPostIt
ID: 39290393
SELECT TOP 10 * FROM QUESTIONS
ORDER BY NEWID()

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Accepted Solution

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jonnidip earned 175 total points
ID: 39292985
Please note that on a  large table (which may not be your case) you may encounter very poor performance by using "order by".
Note that setting the "order by" clause on your query the SQL needs to sort all the rows in tempdb. You can see this by displaying the estimated execution plan (CTRL+L in SSMS).

A solution can be to use the "tablesample" clause after the table name.
This will limit the rows returned, by a number of rows or by percentage.
After that you can still use the "order by newid()", as suggested before:
SELECT TOP 10 * FROM QUESTIONS TABLESAMPLE(1000 ROWS)
ORDER BY NEWID()

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Please check again with the execution plan (check eventually the 2 queries one next to the other)...


Regards.
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LVL 16

Assisted Solution

by:EvilPostIt
EvilPostIt earned 75 total points
ID: 39293413
Just did a quick test on my laptop. 1 million rows table had a structure of id field and question which was a char(200). The query completed in less that 1 second.
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Expert Comment

by:jonnidip
ID: 39293449
@EvilPostIt: I don't want to throw your solution. The poster said that the table had only 100 records... I just wanted to share my opinion over your (correct) solution.
In fact, I have tested your (correct and known as a standard) query on a table with over than 100 million rows and the time difference between the 2  queries (yours and the one I posted, with the addition of tablesample) is very high!

@PM150: The first solution is for sure correct. Just please remember that "size does matter"...
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LVL 16

Expert Comment

by:EvilPostIt
ID: 39293641
The only problem with TABLESAMPLE is the fact that it returns a non-random set of results. So if you ask for 1000 it will choose a number rows in sequence and then a random 10 from that sequence.

The "size matters" comment made me laugh though :)
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Author Comment

by:PM150
ID: 39293859
Tablesample is not really random indeed when want a new andom set while playing again. What about using cast(rand()*100 as int) on the row id?
Tks
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Assisted Solution

by:EvilPostIt
EvilPostIt earned 75 total points
ID: 39295874
RAND() would work if you were after a single question as it only produces 1 random number for all rows in the select statement. If you did only want 1 question you could do the following.

SELECT * FROM QUESTIONS
WHERE ID=CAST(RAND()*(SELECT MAX(ID) FROM QUESTIONS) AS INT)

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You want to select 10 questions so using the above supplied statement will work fine in your scenario,
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Expert Comment

by:EvilPostIt
ID: 39299310
Ok then.....
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