Wikipedia
SQLf is a SQL extended with fuzzy set theory application for expressing flexible ( fuzzy) queries to Regular Relational Databases. Between the known extensions proposed to SQL, at the present time, this is the most complete, because it allows the use of diverse fuzzy elements in all the constructions of the language SQL.
SQLf is the only known proposal of flexible query system allowing linguistic quantification over set of rows in queries throw the extension of SQL nesting and partitioning structures with fuzzy quantifiers. It also allows the use of quantifiers to qualify the quantity of search criteria satisfied by single rows. For query evaluation, they have intended several mechanisms. The more important is the one based on the derivation principle that consists in deriving classic queries that produce, given a threshold t, the t-cut of the result of the fuzzy query, so that the additional processing cost of using a fuzzy language is diminished.