Question

I have to extract two random records from a table.
I have implemented something like this inside a stored procedure:

    with tmpTable as(
        SELECT top 500 [columns I need]
              , row_number() over(order by [myColumn]) as rown
        FROM SourceTable
          JOIN [myJoin]
        WHERE [myCondition]
    )
    -- here I extract with an intervall of 10 records: 10, 20, 30, ..., 400, 410, ...
    select * from tmpTable where rown = (1 + FLOOR(50*RAND()))*10

It works great, it extracts a random record among the first 500 records from my source.
But, when the sp is called from the presentation layer (ASP.NET 4.0, SqlClient ADO.NET), eventually the same record is returned twice. Note that the two calls are indipendent from each other.

I guess it depends on the fact that the sp is called twice in few milliseconds and that the random generator creates the same number. In the debug process no duplication occurs: due to the F10 manual steps, I guess, that require more than few milliseconds.

How may I obtain two different records?

EDIT

The answer of Lamak requires some more details. The source table is made up of records of products. Groups of about 10 records differ from each other only for some carachteristics (e.g. color). The records are distributed in such a way:

1 to 10: product 1
11 to 20: product 2
... and so on

So if I get the first two random records it is highly expected that the records will regard the same product. This is why I’m using an intervall of 10 records in the random extraction.

Answer 1

If you are using SQL Server you can just do the following:

SELECT TOP 1 [columns I need]
FROM SourceTable
JOIN [myJoin]
ON [Something]
WHERE [MyCondition]
ORDER BY NEWID()

If you still want to first isolate the records to each product, you can try this:

SELECT TOP 1 *
FROM (  SELECT [columns I need], ROW_NUMBER() OVER(PARTITION BY Product ORDER BY NEWID())  Corr
        FROM SourceTable
        JOIN [myJoin]
        ON [Something]
        WHERE [MyCondition]) A
WHERE Corr = 1
ORDER BY NEWID()

Though you still can choose a record that has the same product of the first one.