How to create fast database queries

Archive for July 28th, 2009

SQL Server: random records avoiding CTE reevaluation

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From Stack Overflow:

I need to get 5 random records from a table plus a further record based on data from the users preferences.

How can I combine the two statements whilst ensuring that no results are repeated (i. e. the favourite is not present in the 5 random records)?

We should select the 6th record so that it would satisfy two conditions: first, it should be preferred by user, second, it should not be among the 5 random records selected earlier.

The problem with random records it that they are, um, random. We usually cannot reproduce 5 random records to apply further conditions on them.

That's where the CTEs would come handy, but there's a little problem.

A CTE, as Microsoft understands them, is just an alias for an online view.

Therefore, each time a CTE is used in a query, it may (or may not) be reevaluated.

It's not a problem for a plain CTEs which are built out of static columns.

However, this is totally a problem when we try to UNION two queries using an undeterministic CTE (like, ordered by NEWID()), as we would do in this case.

Let's create a sample table and see what happens:

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Written by Quassnoi

July 28th, 2009 at 11:00 pm

Posted in SQL Server