Data Integrity
I already mentioned that data integrity is crucial for a relational database. Actually, data integrity rules are an important part of a relational database. An RDBMS has to enforce the rules. By making the rules part of a database, you inform the system what those rules are. With declarative constraints, how they are enforced is up to the system; with procedural code, you define how to implement them. In both cases, you express constraints in terms of predicates.
Relation headers—physical table and view definitions including attribute type definitions, together with declarative and procedural constraints—form a database schema. Now we can summarize what exactly a database schema is. A database schema represents constrained predicates ...
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