The Gap Between Theory & Practice

Most commercial relational query languages are based on the

relational calculus.

However in some respects they go beyond the formal model.

They support:

• aggregate operators,
• sort orders,
• grouping,
• update capabilities.

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Most commercial relational query languages are based on the relational calculus. But they often support features that go beyond the formal model: aggregate operators such as set cardinality, sort orders for displaying the results in some specified order, grouping for gathering similar data before an aggregation, and update capabilities to modify the stored data.