Chapter 5 Relational Algebra and Relational Calculus Pearson

Chapter 5 - Objectives u Meaning of the term relational completeness. u How to form queries in relational algebra. u How to form queries in tuple relational calculus. u How to form queries in domain relational calculus. u Categories of relational DML. Pearson Education © 2009 2

Introduction u Relational algebra and relational calculus are formal languages associated with the relational model. u Informally, relational algebra is a (high-level) procedural language and relational calculus a non-procedural language. u However, formally both are equivalent to one another. u A language that produces a relation that can be derived using relational calculus is relationally complete. Pearson Education © 2009 3

Relational Algebra u Relational algebra operations work on one or more relations to define another relation without changing the original relations. u Both operands and results are relations, so output from one operation can become input to another operation. u Allows expressions to be nested, just as in arithmetic. This property is called closure. Pearson Education © 2009 4

Relational Algebra u Five basic operations in relational algebra: Selection, Projection, Cartesian product, Union, and Set Difference. u These perform most of the data retrieval operations needed. u Also have Join, Intersection, and Division operations, which can be expressed in terms of 5 basic operations. Pearson Education © 2009 5

Relational Algebra Operations Pearson Education © 2009 6

Relational Algebra Operations Pearson Education © 2009 7

Selection (or Restriction) u 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). Pearson Education © 2009 8

Example - Selection (or Restriction) u List all staff with a salary greater than

Projection u col 1, . . . , 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 and eliminating duplicates. Pearson Education © 2009 10

Example - Projection u Produce a list of salaries for all staff, showing only

Union 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. u. R u If R and S have I and J tuples, respectively, union is obtained by concatenating them into one relation with a maximum of (I + J) tuples. Pearson Education © 2009 12

Example - Union u List all cities where there is either a branch office

Set Difference u. R –S – Defines a relation consisting of the tuples that

Example - Set Difference u List all cities where there is a branch office

Intersection 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. u. R u Expressed using basic operations: R S = R – (R – S) Pearson Education © 2009 16

Example - Intersection u List all cities where there is both a branch office

Cartesian product u. R XS – Defines a relation that is the concatenation of

Example - Cartesian product u List the names and comments of all clients who have viewed a property for rent. ( client. No, f. Name, l. Name(Client)) X ( client. No, property. No, comment (Viewing)) Pearson Education © 2009 19

Example - Cartesian product and Selection u Use selection operation to extract those tuples where Client. client. No = Viewing. client. No((Õclient. No, f. Name, l. Name(Client)) (Õclient. No, property. No, comment(Viewing))) Cartesian product and Selection can be reduced to a single operation called a Join. u Pearson Education © 2009 20

Join Operations u Join is a derivative of Cartesian product. u Equivalent to performing a Selection, using join predicate as selection formula, over Cartesian product of the two operand relations. u One of the most difficult operations to implement efficiently in an RDBMS and one reason why RDBMSs have intrinsic performance problems. Pearson Education © 2009 21

Join Operations u Various forms of join operation – Theta join – Equijoin (a

Theta join ( -join) u. 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 (<, , >, , =, ). Pearson Education © 2009 23

Theta join ( -join) u Can rewrite Theta join using basic Selection and Cartesian product operations. R FS = F(R S) u 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. Pearson Education © 2009 24

Example - Equijoin u List the names and comments of all clients who have viewed a property for rent. ( client. No, f. Name, l. Name(Client)) Client. client. No = Viewing. client. No ( client. No, property. No, comment(Viewing)) Pearson Education © 2009 25

Natural join u. R S – An Equijoin of the two relations R and

Example - Natural join u List the names and comments of all clients who have viewed a property for rent. ( client. No, f. Name, l. Name(Client)) ( client. No, property. No, comment(Viewing)) Pearson Education © 2009 27

Outer join u To display rows in the result that do not have matching values in the join column, use Outer join. u. 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. Pearson Education © 2009 28

Example - Left Outer join u Produce a status report on property viewings. property.

Semijoin u. R FS – Defines a relation that contains the tuples of R that participate in the join of R with S. u Can R rewrite Semijoin using Projection and Join: FS = A(R F S) Pearson Education © 2009 30

Example - Semijoin u List complete details of all staff who work at the

Division u. R S – Defines a relation over the attributes C that consists of set of tuples from R that match combination of every tuple in S. u Expressed using basic operations: T 1 C(R) T 2 C((S X T 1) – R) T T 1 – T 2 Pearson Education © 2009 32

Example - Division u Identify all clients who have viewed all properties with three rooms. ( client. No, property. No(Viewing)) ( property. No( rooms = 3 (Property. For. Rent))) Pearson Education © 2009 33

Aggregate Operations u AL(R) – Applies aggregate function list, AL, to R to define a relation over the aggregate list. – AL contains one or more (<aggregate_function>, <attribute>) pairs. u Main aggregate functions are: COUNT, SUM, AVG, MIN, and MAX. Pearson Education © 2009 34

Example – Aggregate Operations u How many properties cost more than £ 350 per

Grouping Operation u GA AL(R) – Groups tuples of R by grouping attributes, GA, and then applies aggregate function list, AL, to define a new relation. – AL contains one or more (<aggregate_function>, <attribute>) pairs. – Resulting relation contains the grouping attributes, GA, along with results of each of the aggregate functions. Pearson Education © 2009 36

Example – Grouping Operation u Find the number of staff working in each branch and the sum of their salaries. R(branch. No, my. Count, my. Sum) branch. No COUNT staff. No, SUM salary (Staff) Pearson Education © 2009 37

Relational Calculus u Relational calculus query specifies what is to be retrieved rather than how to retrieve it. – No description of how to evaluate a query. u In first-order logic (or predicate calculus), predicate is a truth-valued function with arguments. u When we substitute values for the arguments, function yields an expression, called a proposition, which can be either true or false. Pearson Education © 2009 38

Relational Calculus u If predicate contains a variable (e. g. ‘x is a member of staff’), there must be a range for x. u When we substitute some values of this range for x, proposition may be true; for other values, it may be false. u When applied to databases, relational calculus has forms: tuple and domain. Pearson Education © 2009 39

Tuple Relational Calculus u Interested in finding tuples for which a predicate is true. Based on use of tuple variables. u Tuple variable is a variable that ‘ranges over’ a named relation: i. e. , variable whose only permitted values are tuples of the relation. u Specify range of a tuple variable S as the Staff relation as: Staff(S) u To find set of all tuples S such that P(S) is true: {S | P(S)} Pearson Education © 2009 40

Tuple Relational Calculus - Example u To find details of all staff earning more than £ 10, 000: {S | Staff(S) S. salary > 10000} u To find a particular attribute, such as salary, write: {S. salary | Staff(S) S. salary > 10000} Pearson Education © 2009 41

Tuple Relational Calculus u Can use two quantifiers to tell how many instances the predicate applies to: – Existential quantifier $ (‘there exists’) – Universal quantifier (‘for all’) variables qualified by or $ are called bound variables, otherwise called free variables. u Tuple Pearson Education © 2009 42

Tuple Relational Calculus u Existential quantifier used in formulae that must be true for at least one instance, such as: Staff(S) ($B)(Branch(B) (B. branch. No = S. branch. No) B. city = ‘London’) u Means ‘There exists a Branch tuple with same branch. No as the branch. No of the current Staff tuple, S, and is located in London’. Pearson Education © 2009 43

Tuple Relational Calculus u Universal quantifier is used in statements about every instance, such as: ( B) (B. city ‘Paris’) u Means ‘For all Branch tuples, the address is not in Paris’. u Can also use ~($B) (B. city = ‘Paris’) which means ‘There are no branches with an address in Paris’. Pearson Education © 2009 44

Tuple Relational Calculus u Formulae should be unambiguous and make sense. u A (well-formed) formula is made out of atoms: » R(Si), where Si is a tuple variable and R is a relation » Si. a 1 q Sj. a 2 » Si. a 1 q c u Can recursively build up formulae from atoms: » An atom is a formula » If F 1 and F 2 are formulae, so are their conjunction, F 1 F 2; disjunction, F 1 F 2; and negation, ~F 1 » If F is a formula with free variable X, then ($X)(F) and ( X)(F) are also formulae. Pearson Education © 2009 45

Example - Tuple Relational Calculus u List the names of all managers who earn more than £ 25, 000. {S. f. Name, S. l. Name | Staff(S) S. position = ‘Manager’ S. salary > 25000} u List the staff who manage properties for rent in Glasgow. {S | Staff(S) ($P) (Property. For. Rent(P) (P. staff. No = S. staff. No) P. city = ‘Glasgow’)} Pearson Education © 2009 46

Example - Tuple Relational Calculus u List the names of staff who currently do not manage any properties. {S. f. Name, S. l. Name | Staff(S) (~($P) (Property. For. Rent(P) (S. staff. No = P. staff. No)))} Or {S. f. Name, S. l. Name | Staff(S) (( P) (~Property. For. Rent(P) ~(S. staff. No = P. staff. No)))} Pearson Education © 2009 47

Example - Tuple Relational Calculus u List the names of clients who have viewed a property for rent in Glasgow. {C. f. Name, C. l. Name | Client(C) (($V)($P) (Viewing(V) Property. For. Rent(P) (C. client. No = V. client. No) (V. property. No=P. property. No) P. city =‘Glasgow’))} Pearson Education © 2009 48

Tuple Relational Calculus u Expressions can generate an infinite set. For example: {S | ~Staff(S)} u To avoid this, add restriction that all values in result must be values in the domain of the expression. Pearson Education © 2009 49

Domain Relational Calculus u Uses variables that take values from domains instead of tuples of relations. u If F(d 1, d 2, . . . , dn) stands for a formula composed of atoms and d 1, d 2, . . . , dn represent domain variables, then: {d 1, d 2, . . . , dn | F(d 1, d 2, . . . , dn)} is a general domain relational calculus expression. Pearson Education © 2009 50

Example - Domain Relational Calculus u Find the names of all managers who earn more than £ 25, 000. {f. N, l. N | ($s. N, posn, sex, DOB, sal, b. N) (Staff (s. N, f. N, l. N, posn, sex, DOB, sal, b. N) posn = ‘Manager’ sal > 25000)} Pearson Education © 2009 51

Example - Domain Relational Calculus u List the staff who manage properties for rent in Glasgow. {s. N, f. N, l. N, posn, sex, DOB, sal, b. N | ($s. N 1, cty)(Staff(s. N, f. N, l. N, posn, sex, DOB, sal, b. N) Property. For. Rent(p. N, st, cty, pc, typ, rms, rnt, o. N, s. N 1, b. N 1) (s. N=s. N 1) cty=‘Glasgow’)} Pearson Education © 2009 52

Example - Domain Relational Calculus u List the names of staff who currently do not manage any properties for rent. {f. N, l. N | ($s. N) (Staff(s. N, f. N, l. N, posn, sex, DOB, sal, b. N) (~($s. N 1) (Property. For. Rent(p. N, st, cty, pc, typ, rms, rnt, o. N, s. N 1, b. N 1) (s. N=s. N 1))))} Pearson Education © 2009 53

Example - Domain Relational Calculus u List the names of clients who have viewed a property for rent in Glasgow. {f. N, l. N | ($c. N, c. N 1, p. N 1, cty) (Client(c. N, f. N, l. N, tel, p. T, m. R) Viewing(c. N 1, p. N 1, dt, cmt) Property. For. Rent(p. N, st, cty, pc, typ, rms, rnt, o. N, s. N, b. N) (c. N = c. N 1) (p. N = p. N 1) cty = ‘Glasgow’)} Pearson Education © 2009 54

Domain Relational Calculus u When restricted to safe expressions, domain relational calculus is equivalent to tuple relational calculus restricted to safe expressions, which is equivalent to relational algebra. u Means every relational algebra expression has an equivalent relational calculus expression, and vice versa. Pearson Education © 2009 55

Other Languages u Transform-oriented languages are non-procedural languages that use relations to transform input data into required outputs (e. g. SQL). u Graphical languages provide user with picture of the structure of the relation. User fills in example of what is wanted and system returns required data in that format (e. g. QBE). Pearson Education © 2009 56

Other Languages u 4 GLs can create complete customized application using limited set of commands in a user-friendly, often menu-driven environment. u Some systems accept a form of natural language, sometimes called a 5 GL, although this development is still at an early stage. Pearson Education © 2009 57