Database Management Systems · topic
Relations as sets of tuples, degree and cardinality; superkeys, candidate, primary, alternate and foreign keys with superkey counting; domain, NOT NULL, entity, key, referential and CHECK constraints; which operations can violate each one, and the referential actions CASCADE, SET NULL, SET DEFAULT, RESTRICT and NO ACTION.
An ER diagram becomes a relational schema by a short list of mechanical rules: one table per strong entity set, one per weak entity set, one per many-to-many relationship, one per multivalued attribute, and foreign keys for everything else.
The relational model stores all data as relations: tables whose rows are tuples of atomic values. It was designed so that queries could be expressed as operations on sets, independent of how the rows are laid out on disk.
An integrity constraint is a condition that every legal database instance must satisfy. The DBMS checks it on every insert, delete and update, and rejects (or repairs) any change that would break it.
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