DBMS/Query Languages

Query Languages

query

the retrieval of tuples from the relations of a relational schema

query language

a language used to retrieve information (tuples) from the relations of a relational schema.

types of query language

• procedural language
• non-procedural language

Mathematical query languages

• Relational algebra
• Tuple relational calculus
• Domain relational calculus

Relational algebra

relational algebra is a procedural query language

• • operations
    1. selection (r σ s) -
    2. projection (r ∏ s) -
    3. union: (r ∪ s)
    4. set difference: (r – s)
    5. Cartesian product: (r X s)
    6. rename: ( ρr )
    7. intersection:
    8. natural join:
    9. division:
    10. union:
    11. intersection:
    12. outer join

BY:NA

Tuple relational calculus

A tuple variable is variable that takes on tuples of a particular relation schema as values. That is, every value assigned to a given tuple variable has the same number and type of fields. A tuple relation calculus query has the form {T I p(t)} where T is a tuple variable and p(T) denotes a formula that describes T; The result of this query is the set of all tuples t for which the formula p(T)evaluates to true with T=t.