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Chapter 6: Problem 14

When is a query language called relationally complete?

Short Answer

Expert verified
A query language is called relationally complete if it can perform selection, projection, union, difference, product (Cartesian product), and renaming, as defined in relational algebra; it must be able to express any query that could be expressed in relational algebra.

Step by step solution

01

Define Relational Completeness

Relational Completeness is a term used in Database System to indicate whether a database's query language is capable of expressing all basic retrievals and updates. When applied to a query language, this means the language can be used to manipulate the data in ways defined by the relational model.
02

Specify The Criteria

Codd's Theorem, formulated by Edgar F. Codd, the inventor of the relational model, provides a criterion for relational completeness. According to Codd's theorem, a language is relationally complete if it is at least as powerful as the algebra of sets operationally.
03

Explanation

In other words, if the query language can perform selection, projection, union, difference, product (also known as Cartesian product), and renaming operations, as defined in relational algebra, it qualifies as being relationally complete. It should be able to express any query that can be expressed in relational algebra.

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Most popular questions from this chapter

In a tuple relational calculus query with \(n\) tuple variables, what would be the typical minimum number of join conditions? Why? What is the effect of having a smaller number of join conditions?

Define the following terms with respect to the domain calculus: domain variable, range relation, atom, formula, and expression.

Consider this query: Retrieve the ssns of employees who work on at least those projects on which the employee with \(\operatorname{ss} N=123456789\) works. This may be stated \(\operatorname{as}(\text { FORALL } x)(\text { IF } P \text { THEN } Q),\) where \(\bullet\) \(x\) is a tuple variable that ranges over the PROJECT relation. \(\bullet\) \(P \equiv\) employee with \(\operatorname{ssN}=123456789\) works on project \(x\) \(\bullet\) \(Q \equiv\) employee e works on project \(x\) Express the query in tuple relational calculus, using the rules \(\bullet\) \((\forall x)(P(x)) \equiv \operatorname{NOT}(\exists x)(\operatorname{NOT}(P(x)))\) \(\bullet\) (IF \(P \text { THEN } Q) \equiv(\mathrm{NOT}(P) \text { OR } Q)\)

Discuss the meanings of the existential quantifier \((\exists)\) and the universal quantifier \((\forall)\)

How does tuple relational calculus differ from domain relational calculus?
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