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Random sample of 100 records

Posted on 2014-04-21
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Last Modified: 2014-04-25
How could I get a random sample of 100 records from a very large table?
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Question by:hrolsons
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7 Comments
 
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Jim Horn earned 500 total points
ID: 40013616
Not sure about the 'very large table' part, but otherwise...

SELECT TOP 100 * FROM your_table
ORDER BY NEWID()
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by:Jim Horn
ID: 40014972
Tell you what ... How about telling us the business problem that you're trying to tackle, and maybe we'll be able to come up with a better solution.
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Expert Comment

by:ProjectChampion
ID: 40015214
Since 2008 R2, SQL Server has a built in feature for this puprpose, i.e. TABLESAMPLE. For instance:

USE AdventureWorks2008R2 ;
GO
SELECT FirstName, LastName
FROM Person.Person
TABLESAMPLE (10 PERCENT) ;
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Author Comment

by:hrolsons
ID: 40015520
@Jim Horn - I've hired someone to edit photographs for me and I want them to edit a random sample of my whole collection to see how they do.  I didn't just want to send him 100 of the same track meet.

@ProjectChampion - How do you apply TABLESAMPLE to a fixed number, like 100.
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Expert Comment

by:Anthony Perkins
ID: 40015598
TABLESAMPLE was introduced with SQL Server 2005 and the syntax is:
TABLESAMPLE [SYSTEM] (sample_number [ PERCENT | ROWS ] )

So in your case:
TABLESAMPLE (100 ROWS)
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Expert Comment

by:Anthony Perkins
ID: 40015612
Having said that TABLESAMPLE is approximate, so if you want exactly 100 you would be better off with Jim's solution.
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Expert Comment

by:Anthony Perkins
ID: 40015632
And on second thoughts and after doing some testing with TABLESAMPLE (perhaps I should have done that in the first place) the results are not very random at all (which I believe that is akin to saying that someone is not very pregnant :) )

In fact SQL Server's BOL states:
The sample does not have to be a truly random sample at the level of individual rows.
...
If you really want a random sample of individual rows, modify your query to filter out rows randomly, instead of using TABLESAMPLE. For example, the following query uses the NEWID function to return approximately one percent of the rows of the Sales.SalesOrderDetail table:
...
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