Relational Algebra and Relational Calculus

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Chapter 4

• Relational Algebra and Relational
  Calculus

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Introduction

• Relational algebra and relational calculus are
  formal query languages of the relational model.
• Relational algebra is a procedural language.
• Relational calculus is a non-procedural language.
• Both are equivalent to one another.
• Both have formal strong foundation on logic.
• Query languages ! programming languages
• A language that produces a relation that can be
  derived using relational calculus is relationally
  complete.

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Relational Algebra

• Relational algebra operations work on one or more
  relations to define another relation without
  changing the original relations.
• Set language All tuples are manipulated without
  looping
• Both operands and results are relations, so
  output from one operation can become input to
  another operation.
• Allows expressions to be nested, just as in
  arithmetic. This property is called closure.

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Relational Algebra

• 5 basic operations in relational algebra
  Selection, Projection, Cartesian product, Union,
  and Set Difference.
• These perform most of the data retrieval
  operations needed.
• Derived operations Intersection, Join, Semijoin,
  and Division operations.
• They can be expressed in terms of 5 basic
  operations.

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Relational Algebra Operations
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Relational Algebra Operations
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Selection (or Restriction)

• ?predicate (R)
• Works on a single relation R and defines a
  relation that contains only those tuples (rows)
  of R that satisfy the specified condition
  (predicate).
• No duplicates in the result (Why?)
• Schema of result is identical to the schema of
  the input relation.

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Example - Selection (or Restriction)

• List all staff with a salary greater than
  10,000.
• ?salary gt 10000 (Staff)

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Projection

• ?col1, . . . , coln(R)
• Works on a single relation R and defines a
  relation that contains a vertical subset of R,
  extracting the values of specified attributes.
• It eliminates duplicates.
• Some DBMSs do not eliminate duplicates, unless
  the user asks for it
• Schema of results contains the fields in the
  projection list.

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Example - Projection

• Produce a list of salaries for all staff, showing
  only staffNo, fName, lName, and salary details.
• ?staffNo, fName, lName, salary(Staff)

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Union

• R ? S
• Union of two relations R and S defines a relation
  that contains all the tuples of R, or S, or both
  R and S, duplicate tuples being eliminated.
• R and S must be union-compatible.
• Same number of fields
• Corresponding fields have the same domain
• Schema of the result is 1st relation schema.

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Example - Union

• List all cities where there is either a branch
  office or a property for rent.
• ?city(Branch) ? ?city(PropertyForRent)

?

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Set Difference

• R S
• Defines a relation consisting of the tuples that
  are in relation R, but not in S.
• R and S must be union-compatible.
• Schema of the result is 1st relation schema.
• Same definition applies to all the set operations.

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Example - Set Difference

• List all cities where there is a branch office
  but no properties for rent.
• ?city(Branch) ?city(PropertyForRent)

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Intersection

• R ? S
• Defines a relation consisting of the set of all
  tuples that are in both R and S.
• R and S must be union-compatible.
• Expressed using basic operations
• R ? S R (R S)

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Example - Intersection

• List all cities where there is both a branch
  office and at least one property for rent.
• ?city(Branch) ? ?city(PropertyForRent)

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Cartesian product

• R X S
• Defines a relation that is the concatenation of
  every tuple of relation R with every tuple of
  relation S.
• Each row of R is paired with each row of S.
• Result schema has one field for each field of R
  and S.

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Example - Cartesian Product

• List the names and comments of all clients who
  have viewed a property for rent.
• (?clientNo, fName, lName(Client)) X (?clientNo,
  propertyNo,comment (Viewing))

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Example Cartesian Product and Selection

• Use selection operation to extract those tuples
  where Client.clientNo Viewing.clientNo.
• sClient.clientNo viewing.clientNo((ÕclientNo,fNa
  me,lName(Client)) ? (ÕclientNo,propertyNo,comment(
  Viewing)))

• Cartesian product and Selection can be reduced
  to a single operation called a Join.

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Join Operations

• Join is a derivative of Cartesian product.
• Equivalent to performing a Selection, using join
  predicate as selection formula, over Cartesian
  product of the two operand relations.
• One of the most difficult operations to implement
  efficiently in an RDBMS and one reason why RDBMSs
  have intrinsic performance problems.

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Join Operations

• Various forms of join operation
• Theta join
• Equijoin (a particular type of Theta join)
• Natural join
• Outer join
• Semijoin

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Theta join (?-join)

• R FS
• Defines a relation that contains tuples
  satisfying the predicate F from the Cartesian
  product of R and S.
• The predicate F is of the form R.ai ? S.bi where
  ? may be one of the comparison operators (lt, ?,
  gt, ?, , ?).

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Theta join (?-join)

• Can rewrite Theta join using basic Selection and
  Cartesian product operations.
• R FS ?F(R ? S)

• Degree of a Theta join is sum of degrees of the
  operand relations R and S. If predicate F
  contains only equality (), the term Equijoin is
  used.

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Example - Equijoin

• List the names and comments of all clients who
  have viewed a property for rent.
• (?clientNo,fName,lName(Client))
  Client.clientNo Viewing.clientNo
  (?clientNo,propertyNo,comment(Viewing))

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Natural Join

• R S
• An Equijoin of the two relations R and S over all
  common attributes x. One occurrence of each
  common attribute is eliminated from the result.

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Example - Natural Join

• List the names and comments of all clients who
  have viewed a property for rent.
• (?clientNo,fName,lName(Client))
  (?clientNo,propertyNo,comment(Viewing))

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Another example of a Natural join

• Identify all clients who have viewed properties
  with three or four rooms.
• ?clientNo((Viewing) (?propertyNo(?rooms 3
  or rooms 4 (PropertyForRent))))
• What happens if or is replaced by and ?

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Outer join

• To display rows in the result that do not have
  matching values in the join column, use Outer
  join.
• R S
• (Left) outer join is join in which tuples from R
  that do not have matching values in common
  columns of S are also included in result relation.

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Example - Left Outer join

• Produce a status report on property viewings.
• ?propertyNo,street,city(PropertyForRent)
  Viewing

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Semijoin

• R FS
• Defines a relation that contains the tuples of R
  that participate in the join of R with S.
• Result schema is the schema of the first relation.

• Can rewrite Semijoin using Projection and Join
• R FS ?A(R F S)

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Example - Semijoin

• List complete details of all staff who work at
  the branch in Glasgow.
• Staff Staff.brancNo Branch.branchNo and
  branch.city Glasgow Branch

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Division

• R ? S
• Not supported as a primitive operator
• Let R have 2 fields, x and y S has only field y
• R ? S is the set of all x values in R such that
  the y
• values associated with an x value in R
  contains all y
• values in S.
• In general, x and y can be any lists of fields.
• Expressed using basic operations
• T1 ? ?x(R)
• T2 ? ?x((S X T1) R)
• T ? T1 T2

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Example - Division

• Identify all clients who have viewed all
  properties with three rooms.
• (?clientNo,propertyNo(Viewing)) ?
  (?propertyNo(?rooms 3 (PropertyForRent)))

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Relational Calculus

• Two versions tuple relational calculus (TRC) and
  domain relational calculus (DRC).
• Calculus uses variables, constants, operators
  (comparison and logical), and quantifiers.
• TLC variables range over tuples (tuple variable)
• DRC variables range over domain elements (domain
  variable).
• Expressions in relational calculus are called
  formulas (or predicates).
• An answer to a formula is a set of tuples that
  make the formula evaluate to true.

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Tuple Relational Calculus

• Query has the form S p(S)
• S is a tuple variable and p(S) is a formula.
• It finds the set of all tuples S such that P(S)
  is true.
• Tuple variable is a variable that ranges over a
  named relation ie., variable whose only
  permitted values are tuples of the relation.
• Specify range of a tuple variable S as the Staff
  relation as Staff(S)
• S Staff(S) ? Get all tuples of Staff
  relation

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Example - Tuple Relational Calculus

• To find details of all staff earning more than
  10,000
• S Staff(S) ? S.salary gt 10000
• To find a particular attribute, such as salary,
  write
• S.salary Staff(S) ? S.salary gt 10000
• Queries are evaluated on instances of Staff.

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Tuple Relational Calculus

• Can use two quantifiers to tell how many
  instances the predicate applies to
• Existential quantifier (there exists)
• Universal quantifier " (for all)
• Tuple variables qualified by " or are called
  bound variables, otherwise called free variables.
• Variables to the left of must be the only
  free variables in the formula p().
• Otherwise, the answer is either T or F

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Tuple Relational Calculus

• Existential quantifier used in formula that must
  be true for at least one instance, such as
• Staff(S) Ù (B)(Branch(B) Ù (B.branchNo
  S.branchNo) Ù B.city London)
• Means There exists a Branch tuple that has the
  same branchNo as the branchNo of the current
  Staff tuple, S, and is located in London.

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Tuple Relational Calculus

• Universal quantifier is used in statements about
  every instance, such as
• ("B) (B.city ? Paris)
• Means For all Branch tuples, the city is not
  Paris.
• Can also use (B) (B.city Paris) which means
  There are no branches in Paris.
• ("B) (P(B)) ? (B) (P(B))

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Tuple Relational Calculus

• A (well-formed) formula is made out of atoms
• R(Si), where Si is a tuple variable and R is a
  relation
• Si.a1 q Sj.a2, where ai is an attribute
• Si.a1 q c , where c is a constant
• Can recursively build up formulas from atoms
• An atom is a formula
• If F1 and F2 are formulas, so are their
  conjunction, F1 Ù F2 disjunction, F1 Ú F2 and
  negation, F1
• If F is a formula with free variable X, then
  (X)(F) and ("X)(F) are also formulas.

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Example - Tuple Relational Calculus

• List the names of all managers who earn more than
  25,000.
• S.fName, S.lName Staff(S) ?
• S.position Manager ? S.salary gt 25000
• List the staff who manage properties for rent in
  Glasgow.
• S Staff(S) ? (P) (PropertyForRent(P) ?
  (P.staffNo S.staffNo) Ù P.city Glasgow)

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Example - Tuple Relational Calculus

• List the names of staff who currently do not
  manage any properties.
• S.fName, S.lName Staff(S) ? ((P)
  (PropertyForRent(P)?(S.staffNo P.staffNo)))
• Or
• S.fName, S.lName Staff(S) ? (?P)
  (PropertyForRent(P) ?
• (S.staffNo P.staffNo)))

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Example - Tuple Relational Calculus

• List the names of clients who have viewed a
  property for rent in Glasgow.
• C.fName, C.lName Client(C) Ù ((V)(P)
• (Viewing(V) Ù PropertyForRent(P) Ù (
• C.clientNo V.clientNo) Ù
• (V.propertyNoP.propertyNo)ÙP.city Glasgow))