1. A method for executing a query, comprising: receiving the query at a computer system, wherein the query applies a one or more qualitative search terms to an attribute of data items in a set of data items; and while executing the query on the computer system, processing each data item in the set of data items by, extracting an attribute value from the data item, using a concept-mapping to determine a compatibility index for the attribute value, wherein the concept-mapping associates each attribute value with a numerical compatibility index that indicates a compatibility between the attribute value and the one or more qualitative search terms, wherein the concept mapping is represented using an array containing X and Y coordinate pairs describing a shape of a concept-mapping function, wherein using the concept-mapping to determine the compatibility index includes using the attribute value to perform a lookup in the array to retrieve the compatibility index, and using the compatibility index as a factor in determining whether to include the data item in a set of query results; and wherein when the query includes multiple qualitative search terms, executing the query comprises, determining compatibility indices for the multiple qualitative search terms for each data item, combining the determined compatibility indices into an aggregate compatibility index for each data item, wherein combining the determined compatibility indices involves one of computing an average for the multiple compatibility indices, and computing a weighted average for the multiple compatibility indices based on an ordering of associated qualitative search terms in the query, and determining whether to include each data item in the set of query results based on whether the aggregate compatibility index for the data item meets or exceeds a threshold.