Relational algebra and query meaning
Research context
postgraduate concept · established · As of 2026-10-10
A relation is a set of tuples over a schema.
Key knowledge
A relation is a set of tuples over a schema. Selection keeps rows satisfying a predicate, projection selects attributes and removes duplicates under set semantics, and join combines tuples satisfying a stated condition. SQL normally has bag semantics and three-valued NULL logic, so algebraic rewrites must retain duplicate, NULL and ordering assumptions. An algebraic equivalence describes results, not a fixed execution cost.
Conditions and scope
- Classical algebra here uses finite set-valued relations; SQL implementations must specify bag/NULL semantics.
- A query plan's cost also depends on indexes, cardinality, data distribution and storage.
Sources · 1
References
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Evidence and review
Author checked this English statement, displayed equation where present and stated hypotheses against these source locations: Relational model and algebra: selection, projection, union and joins. The check is bounded to the recorded claims; no complete proof reconstruction, software proof replay or universal literature completeness is claimed.
2026-10-10 · Author source-body/statement/equation/domain check, with graph associations scoped individually. Separate independent content/source/inventory review is required before branch closure.
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