Explain the following terms:
? relationally complete;
? closure of relational operations.
A language that can be used to produce any relation that can be derived using the
relational calculus is said to be relationally complete.
The relational algebra is a theoretical language with operations that work on one or more
relations to define another relation without changing the original relation(s). Thus, both
the operands and the results are relations, and so the output from one operation can
become the input to another operation. This allows expressions to be nested in the
relational algebra, just as we can nest arithmetic operations. This property is called
closure: relations are closed under the algebra, just as numbers are closed under
arithmetic operations.
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