1. A method of estimating probabilities in a relational database, comprising: applying a relational algebra operator to the database, the operator acting to: identify a list of records in the database, each record containing first and second attribute values; identify all the second attribute values found in the list of records; and assign a probability to each first attribute value on the basis of the frequency of its occurrence relative to each second attribute value, wherein the probabilities for the first attribute values are normalized so that the sum of the probabilities is 1.