MRDS provided a commandlevel interface for defining databases and views called data submodels, and a calllevel interface for queries and data manipulation. A separate Logical Inquiry and Update System LINUS provided an online query and update interface. The MRDS query language was similar to SEQUEL as SQL was first called, with range, select, and where clauses corresponding approximately to the FROM, SELECT, and WHERE clauses of SQL. Explicit set operations intersection, union, and difference were provided; there was no direct sorting support. A query was passed as a character string to the MRDS at runtime; there was no precompilation mechanism. Concurrent access to a database by multiple processes was supported; each process was required to explicitly declare the type of access retrieval or update and, for update, the scope set of relations of the update. The database could be quiesced and backed up in its entirety. A transaction mechanism for atomically committing multiple updates was added in a later release.

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