The PRISMA/DB system contains a parser to translate the database language SQL into eXtended Relational Algebra (XRA). The early definition of XRA, which has a multi-set semantics, proves inadequate for translating SQL according to its nested-iteration semantics. The prime cause is that no distinction is made between original and generated duplicate tuples during nested query handling. To achieve a correct translation, tuple identifiers were introduced into XRA and the system.

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PDS Workshop
Centrum Wiskunde & Informatica, Amsterdam (CWI), The Netherlands

Verbrugge, N.Th. (1990). Duplicates and translation of nested SQL queries into XRA. In Lecture Notes in Computer Science/Lecture Notes in Artificial Intelligence (pp. 354–364). doi:10.1007/3-540-54132-2_66