A topic related to SQO is that of cooperative answering systems.
SQO, user constraints, and cooperative answering systems are important contributions both for relational database and DDB systems.
Indeed, I cannot imagine a DDB developed for commercial systems to be successful if it does not contain both SQO and cooperative answering capabilities.
The SQO method described in McSkimin and Minker (1977) was incorporated into the system.
Integrity constraints were used to perform SQO on queries.
A further development is that SQO is beginning to be incorporated into relational databases.
With the introduction of recursion and SQO techniques into relational database technology, it will be necessary to provide users with cooperative responses so they understand why certain queries fail or succeed.
Recursive views that use the magic set technique for implementation are being permitted, and methods developed for SQO are being applied.
SQO can handle either case and inform the user which constraint might be applicable to a query.