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CM036: Advanced Database. Lecture 3 [Self Study] Relational Calculus. Relational Calculus. Declarative language based on predicate logic - checks what is true in the database rather then looking to get it

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cm036 advanced database

CM036: Advanced Database

Lecture 3 [Self Study]

Relational Calculus

relational calculus
Relational Calculus
  • Declarative language based on predicate logic - checks what is true in the database rather then looking to get it
  • The main difference in comparison with the relational algebra is the introduction of variables which range over attributes or tuples
  • Two different variations of the relational calculus:
    • domain calculus - specification of formal properties for different data types, used for describing data
    • tuple calculus - querying and checking formal properties of stored relational data

CM036: Advanced Databases Relational Calculus

predicate calculus
Predicate Calculus
  • Vocabulary of constant names, functional attributes and predicate properties, used for describing the information
  • Phrases for specification of constant, functionally dependant or predicated properties build using proper vocabulary terms
  • Statements about the world, formulated as meaningful phrases, connected through logical connectives in logical sentences
    • conjunction ()
    • disjunction ()
    • negation ()
    • implication ()
    • equivalence ()
  • Possible quantification of the sentences using existential () or universal() quantifiers, ranging over variables

Example:All units registered in the database have unit leaders

among the lecturers

(x).(Unit (x)  (y).(Lecturer(y)  Leader(y,x)))

CM036: Advanced Databases Relational Calculus

relational db as a predicate calculus model
Relational DB as a Predicate Calculus model
  • Semantic interpretation of the calculus is given in a set, so either all type domains of the relational schema (domain calculus), or the set of all relations in the database (tuple calculus) can serve a model for it
  • All meaningful sentences in the model are true or false, so the relation tuples can be interpreted as truthful facts describing the world
  • The constraints describe logical regularities among the attribute values, so they can be expressed as logical sentences

Example:All records of unit leaders have primary keys

    • Relation names are predicates with a place for each attribute

Leader(Lno:INTEGER,Lname:CHAR,Uname:CHAR) - Lno, Lname and Uname are attributes of Leader

    • Data values in the relation tuples are constants from the domains

Leader(2,‘Johnson’,’CS234’) - 2,Johnsonand CS234are constants forming one Leader tuple

    • Constraints can be stated using quantified variables over domains

(x)(y,z) .(Leader(x,y,z)) - x stands for the primary key of Leader

CM036: Advanced Databases Relational Calculus

slide5
Notes:

1. In predicate calculus sentences contain quantified variables only

2. The sub-expressions following the quantified variables form the scope of quantification; it is usually closed in round parentheses

Example:The table for unit leaders has foreign keys to both the lecturers and units tables

  • Relation names are predicates with place for each attribute

Unit(Uno:INTEGER, Uname:CHAR) - Uno and Uname are attributes of relation Unit

Lecturer(Lno:INTEGER, Lname:CHAR) - Lno and Lname are attributes of relation LecturerLeader(LUno:INTEGER,Lno:INTEGER,Uno:INTEGER) - LUno, Lno and Uno are attributes of relation Leader

  • All foreign key constraints can be stated logically using quantified sentences, in which variables range over the respective values

( LUno,Uno,Lno).(Leader(LUno,Uno,Lno) 

( Lname).(Lecturer(Lno,Lname)) 

( Uname).(Unit(Uno,Uname))

)

CM036: Advanced Databases Relational Calculus

relational calculus as d atabase q uery l anguage
Relational Calculus as database query language
  • Queries are logical expressions, which contain non-quantified (free) variables for tuples
  • The free variables are placeholders for the information, we are looking for from the database relations
  • The logical expressions, used in the query, are its conditions which need to be met during answering the query

Example:Who are the employees with salary greater than 40 000?

{e.FNAME,e.LNAME |Employee(e)  e.SALARY > 40 000}

  • An answer is a replacement of the free variables in the query with tuple attributes from the relations used to formulate the query conditions; they should make the statement of the query true in database (pattern matching)

Answer:James Borg and Jennifer Wallace

Free variablesMatchingvalues

e.FNAMEJames Jennifer

e.LNAMEBorg Wallace

CM036: Advanced Databases Relational Calculus

slide7
Notes:

1. The question variables are always listed in front of the

expression, separated by | from the question condition;

2. Question condition can contain free variables only from the list

in the beginning; all other variables should be quantified

  • Querying related tables requires check of the corresponding keys for equality

Example:Retrieve the name and address of all employees

who work for the ‘Research’ department

{e.FNAME,e.LNAME,e.ADDRESS |

Employee(e) 

( d).(Department(d) 

d.DNAME = ‘Research’ 

d.DNUMBER = e.DNO

)

}

CM036: Advanced Databases Relational Calculus

equvalence between relational algebra and relational calculus
Equvalence between relational algebra and relational calculus
  • Relational algebra and relational calculus are equivalent with respect to their ability to formulate queries against relational databases (Codd 1972)
  • Relational algebra concentrates on the procedural (“how-to”)aspects and because of this it is used as an intermediate language for optimization of database queries
  • Relational calculus is more appropriate to specify declaratively the model properties (“what is true”), without worrying about the way it is achieved and as such it can be used as a specification or querying language

Note:The relational language Query-By-Example (QBE), which is developed by IBM during 70ties and is implemented in Paradox and Access desktop database systems is based on the relational calculus

CM036: Advanced Databases Relational Calculus