1 Summary Practical database query languages are usually equipped with some aggregate functions. For example, \\nd mean of column" can be expressed in SQL. However, the manner in which aggregate functions were introduced in these query languages leaves something to be desired. Breazu-Tannen, Buneman, and Wong 3] introduced a nested relational language NRC(=) based on monads 16, 24] and structur...

Introduction The development of electronic information systems and tools will create a demand for new services as well as opportunities for new business areas in the future society. This society, including all the inhabitants, the industries, the service branch and the trade and market organisations , will find it evident to make use of the digital information and related functions in a large n...