Random Weighted Choice in T-SQL

Question

How do you randomly select a table row in T-SQL based on an applied weight for all candidate rows?

For example, I have a set of rows in a table weighted at 50, 25, a

Answer 1

The way to do this with random number generators is to integrate the probabiliity density function. With a set of discrete values you can calculate the prefix sum (sum of all values up to this one) and store it. With this you select the minumum prefix sum (aggregate to date) value greater than the random number.

On a database the subsequent values after an insertion have to be updated. If the relative frequency of updates and size of the data set doesn't make the cost of doing this prohibitive it means that the appropriate value can be obtained in from a single s-argable (predicate that can be resolved by an index lookup) query.