Monday, December 14, 2009

Top Query Revealed: Top Query to find Random Records

Top Query to find Random Records

This is the fourth in a series on the Top Query. In previous posts, I've discussed Simple Top Queries, Aggregate and Grouping Top Queries, and the Problem of Ties in a Top Query.

One rather surprising use for a Top query is to return a resultset of randomly selected records. It's also surprisingly simple. All you really need to do is sort on a random number, or rather, generate a random number in the Order By clause of the query.

So how do you get a random number? Fortunately, Access has a built in function called Rnd that will supply one.

To use Rnd, you need to supply it with a seed or an initial value to generate a pseudorandom number. Any numeric field will do for a seed value, but I generally use an autonumber, primary key field. In the case of the Orders table, that would be OrderID, but OrderNum would have worked as well.

SELECT TOP 10 Account, OrderNum, [Pairs Shipped], [Total Price]
FROM Invoices
ORDER BY Rnd(OrderID);

Every time this query is run, it will return 10 different records, well, sort of. Used as is, the Rnd function will produce a different set of records each time. However, if you close the database and re-open it, it will run the same records in exactly the same sequence.

In order to get a truly random set of numbers each time, you have to add the Randomize command. To do that, you have to create your own function. In a global module, create the following function:

Public Function gRnd(FeedNum As Long) as Double
gRnd = Rnd(FeedNum)
End Function

Then use your user-defined rounding function in your Order By clause:

SELECT TOP 10 Account, OrderNum, [Pairs Shipped], [Total Price]
FROM Invoices
ORDER BY gRnd(OrderID);

A free sample illustrating this process can be found here:

Next time, I'll finish up the Top Query by looking at Creating a Parameter for Top Value.

No comments: