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RELATIONAL ALGEBRA Lecture 2 1

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2 Relational Algebra Operations to manipulate relations. Used to specify retrieval requests (queries). Query results in the form of a relation -> this relation may be used

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3 Theoretical expressions where both operands are relations The output from one operation can become the input to another operation (nested relational algebra) Eight fundamental operations in relational algebra: Unary operations work on one relation Selection Projection Binary operations work on pairs of relations Cartesian productJoin UnionIntersection Set differenceDivision

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4 Algebra Operators abcabc xyxy aabbccaabbcc xyxyxyxyxyxy Cartesian Product a1 a2 a3 b1 b2 b3 b1 b2 b3 c1 c2 c3 a1 a2 a3 b1 b2 b3 c1 c2 c3 Join xyxy a aaabcaaabc xyzxyxyzxy Divide SelectionProjection UnionIntersection Difference

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5 Selection Operation Return the tuples of R that satisfy a selection condition Denoted by (R) Selection condition = can be one condition or more using AND / OR. Example: Select all staff with a salary greater than 10,000 salary > 10000 (STAFF)

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6 SL21 SG37 SG14 SG5 StaffNo John Ann David Susan FName White Beech Ford Brand LNameSex M F M F DOB 1-Oct-45 10-Nov-60 24-May-58 3-Jun-40 Salary 30000 12000 18000 24000 Selection Operation BrnNo D005 D003 salary > 10000 (STAFF)

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7 Projection Operation Return the attributes of R Denoted by (R) Example: Produce a list of salaries for all staff, showing only the staff number, first name, last name, and salary. sno,fname,lname,salary (STAFF)

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8 SL21 SG37 SG14 SNo John Ann David FName 30000 Salary 12000 18000 Projection Operation LName White Ford Beech SA9 SG5 SL41 Mary Susan Julie Howe Lee Brand 9000 24000 9000 sno,fname,lname,salary (STAFF)

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9 Question Retrieve the names and salaries of employees who work in department 4. FNAME,LNAME,SALARY ( DNO=4 (EMPLOYEE)) R1= DNO=4 (EMPLOYEE) R2= FNAME,LNAME,SALARY ( R1) You can rename the attributes of the resulted relation R1= DNO=4 (EMPLOYEE) R2 (FIRSTNAME, LASTNAME,SALARY) = FNAME,LNAME,SALARY ( R1) OR

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10 Set Operations Binary operations Union R S Intersection R S Difference R-S Cartesian Product R x S Note: (union compatible): Dom ( Ai ε R )= Dom (Bi ε S) R,S must have the same number of attributes and same domain. The resulting relation has the same names as the first operand (R)

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11 Union Operation Return a relation that includes all tuples that are either in R or in S, or in both R and S. Eliminate duplicate tuples. R & S must be union-compatible. Denoted by R S R S R S abcabc adfadf abcdfabcdf

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12 Union Operation Example: List all cities where there is either a branch office or a property for rent. city (BRANCH) city (PROPERTY)

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13 PA14 PL94 PG4 PG36 PG21 PropertyNo Holheld Argyll St Lawrence Monar Novar Street Aberdeen London Glasgow Bristol City PostCode AB7 5SU NW2 G11 9QX G32 4QX G12 Type House Flat House OwnerNo PROPERTY B001 B002 B003 BrnNo Deer Rd Argyll St Main St Street SW1 4EH CO46 CO87 CO40 CO93 CO87 PostCode AB2 3SU G11 9QX BRANCH Union Operation City London Glasgow Aberdeen City London Glasgow Aberdeen Bristol city (BRANCH) city (PROPERTY) BrnNo B001 B003

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14 Intersection Operation Return a relation that includes all tuples that are in both R and S. R & S must be union-compatible. Denoted by R S R S R S abcabc adfadf a

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15 Intersection Operation Example: List all cities where there is a branch office and at least one property for rent. city (BRANCH) city (PROPERTY)

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16 PA14 PL94 PG4 PG36 PG21 PropertyNo Holheld Argyll St Lawrence Monar Novar Street Aberdeen London Glasgow Bristol City PostCode AB7 5SU NW2 G11 9QX G32 4QX G12 Type House Flat House OwnerNo PROPERTY B001 B002 B003 BrnNo Deer Rd Argyll St Main St Street SW1 4EH CO46 CO87 CO40 CO93 CO87 PostCode AB2 3SU G11 9QX BRANCH Intersection Operation City London Glasgow Aberdeen City city (BRANCH) city (PROPERTY) London Glasgow Aberdeen BrnNo B001 B003

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17 Difference Operation Return a relation that includes all tuples that are in R but NOT in S. R & S must be union-compatible. Denoted by R - S R S R - S abcabc adfadf bcbc

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18 Difference Operation Example: List all cities where there is a property for rent but no branch office. city (PROPERTY) - city (BRANCH)

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19 PA14 PL94 PG4 PG36 PG21 PropertyNo Holheld Argyll St Lawrence Monar Novar Street Aberdeen London Glasgow Bristol City PostCode AB7 5SU NW2 G11 9QX G32 4QX G12 Type House Flat House OwnerNo PROPERTY B001 B002 B003 BrnNo Deer Rd Argyll St Main St Street SW1 4EH CO46 CO87 CO40 CO93 CO87 PostCode AB2 3SU G11 9QX BRANCH Difference Operation City London Glasgow Aberdeen City Bristol city (PROPERTY) - city (BRANCH) BrnNo B001 B003

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20 Cartesian Product Operation Return a relation that is the concatenation of tuples from two relations R & S Denoted by R X S {a,b} X {1,2,3} = {(a,1), (a,2), (a,3), (b,1), (b,2), (b,3)} Cartesian Product is meaningless on its own, it can combine related tuples from two relations if followed by the appropriate SELECT operation.

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21 Cartesian Product Operation Example: List the names and comments of all clients who viewed a property. Step 1: Cartesian product ClientNo, Fname, Lname (CLIENT) X ClientNo, PrprtyNo, Comment (VIEW) Client.ClientNo, Fname, Lname, View.ClientNo, PrprtyNo, Comment (CLIENT X VIEW) Step 2: Selection Client.ClientNo=View.ClientNo ( ClientNo, Fname, Lname (CLIENT) X ClientNo, PrprtyNo, Comment (VIEW) )

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22 CR76 CO56 CR74 CR62 ClientNo John Aline Mike Mary FName Kay Stewart Ritchie Tregear LNameTelNo 207-774-5632 141-848-1825 1475-392178 1224-196720 CLIENT CR56 CR76 CR56 ClientNo PA14 PG4 PrprtyNo Too small Comment Too remote No dining room VIEW Cartesian Product Operation ViewDate 24-May-01 26-May-01 20-Apr-01 CR62 CR56 PA14 PG3628-Apr-01 14-May-01 Cardinality = 4 Tuples Degree = 3 Attributes (projected) Cardinality = 5 Tuples Degree = 3 Attributes (projected)

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23 CR76 CR56 CR74 CR62 Client. ClientNo John Aline Mike Mary FName Kay Stewart Ritchie Tregear LName View. ClientNo PrprtyNo Comment CR56 CR76 CR56 PA14 PG4 Too small Too remote No dining room CR62 CR56 PA14 PG36 CR76JohnKay CR76JohnKay CR76JohnKay CR76JohnKay CR56 CR76 CR56 PA14 PG4 Too small Too remote No dining room CR62 CR56 PA14 PG36 CR56 CR76 CR56 PA14 PG4 Too small Too remote No dining room CR62 CR56 PA14 PG36 CR56 AlineStewart CR56 AlineStewart CR56 AlineStewart CR74 MikeRitchie CR74 MikeRitchie CR74 Mike Ritchie CR74Mike Ritchie CR74Mike Ritchie CR56 CR76 CR56 PA14 PG4 Too small Too remote No dining room CR62 CR56 PA14 PG36 CR62 Mary Tregear CR62 Mary Tregear CR62 Mary Tregear CR62 Mary Tregear Cardinality = 5 * 4 = 20 Tuples Degree = 3 + 3 = 6 Attributes ClientNo, Fname, Lname (CLIENT) X clientNo, PropertyNo, comment (VIEW))

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24 Join Operation Return a relation that is the concatenation of tuples from two relations R & S that satisfy a certain condition Form of join operation: - Theta join - Equijoin - Natural join - Outer join

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25 * Similar to CP followed by Select. The condition is called join condition (>,<, = ….) The join condition contains one or more equality conditions. (Could be redundant) Contains one or more equality conditions With no redundancy If there are no matching tuples in S that matches R then the NULL is shown

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27 Theta join Operation Return a relation that includes all tuples that satisfy the logical condition F from the Cartesian product of R & S. Logical operators ( ) Denoted by R F S = F (R X S)

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28 Theta join Operation CLIENT(ClientNo, FName, Lname, TelNo) VIEW(ClientNo, PropertyNo, ViewDate, Comment) Example: List the names and comments of all renters who have viewed a property. Client.ClientNo, Fname, Lname, View.ClientNo, PrprtyNo, Comment (CLIENT Client.ClientNo=View.ClientNo VIEW) Alternative: Client.ClientNo=View.ClientNo ( ClientNo, Fname, Lname (CLIENT) X ClientNo, PrprtyNo, Comment (VIEW))

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29 Equijoin Operation A Theta join where the logical condition is equality (=). Example: List the names and comments of all renters who have viewed a property. Client.ClientNo, Fname, Lname, View.ClientNo, PrprtyNo, Comment (CLIENT Client.ClientNo=View.ClientNo VIEW)

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30 CR76 CR56 CR74 CR62 Client. ClientNo John Aline Mike Mary FName Kay Stewart Ritchie Tregear LName View. ClientNo PrprtyNo Comment CR56 CR76 CR56 PA14 PG4 Too small Too remote No dining room CR62 CR56 PA14 PG36 CR76JohnKay CR76JohnKay CR76JohnKay CR76JohnKay CR56 CR76 CR56 PA14 PG4 Too small Too remote No dining room CR62 CR56 PA14 PG36 CR56 CR76 CR56 PA14 PG4 Too small Too remote No dining room CR62 CR56 PA14 PG36 CR56 AlineStewart CR56 AlineStewart CR56 AlineStewart CR74 MikeRitchie CR74 MikeRitchie CR74 Mike Ritchie CR74Mike Ritchie CR74Mike Ritchie CR56 CR76 CR56 PA14 PG4 Too small Too remote No dining room CR62 CR56 PA14 PG36 CR62 Mary Tregear CR62 Mary Tregear CR62 Mary Tregear CR62 Mary Tregear ClientNo, Fname, Lname (CLIENT) X clientNo, PropertyNo, comment (VIEW))

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31 CR76 CR56 Client. ClientNo John Aline FName Kay Stewart LName View. ClientNo PrprtyNo Comment CR76 CR56 PG4 PA14 Too remote Too small PG4 PA14CR62 No dining roomMary Tregear CR62 Client.ClientNo=View.ClientNo ( ClientNo, Fname, Lname (CLIENT) X ClientNo, PrprtyNo, Comment (VIEW)) CR56 AlineStewart Equijoin Operation

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32 Natural join Operation Equijoin of two relation R & S over all common attributes and values. One occurrence of each common attribute is eliminated from the result Denoted by R * (A ε R ),(B ε S ) S Example: Retrieve each EMPLOYEE’s name and the name of department he/she belongs to: T EMPLOYEE * (DNO),(DUNMBER) DEPARTMENT RESULT π FNAME, LNAME, DNAME (T)

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33 Natural join Operation equality test Invariably the JOIN involves an equality test, and thus is often described as an equi-join. Such joins result in two attributes in the resulting relation having exactly the same value. A `natural join' will remove the duplicate attribute(s). If you do use natural joins make sure that the relations do not have two attributes with the same name by accident.

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34 Outer join Operation A natural join in which tuples from R that do not have matching values in the common attributes of S are included in the result relation. Missing values in S are set to null. Denoted by R S Example: Produce a status report on property viewings. ( PropertyNo, Street, City, CLientNo,ViewDate, Comment (PROPERTY) VIEW)

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35 PA14 PL94 PG4 PG36 PG21 PropertyNo Holheld Argyll St Lawrence Monar Novar Street Aberdeen London Glasgow Bristol City PostCode AB7 5SU NW2 G11 9QX G32 4QX G12 Type House Flat House OwnerNo PROPERTY CO46 CO87 CO40 CO93 CO87 VIEW Outer Join Operation CR56 CR76 CR56 ClientNo PA14 PG4 PrprtyNo Too small Comment Too remote No dining room ViewDate 24-May-01 26-May-01 20-Apr-01 CR62PA14 14-May-01

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36 PA14 PL94 PG4 PG36 PG21 PropertyNo Holheld Argyll St Lawrence Monar Novar Street Aberdeen London Glasgow Bristol City ClientNo CR56 null CR76 ViewDate 24-May-01 null 20-Apr-01 Comment Too small null Too remote Outer Join Operation PA14 HolheldAberdeenCR6214-May-01 No dining room null PG4 Lawrence Glasgow CR5626-May-01 null PropertyNo, Street, City, CLientNo,ViewDate, Comment (PROPERTY) VIEW

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37 MORE EXAMPLES

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38 UNION Example

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39 INTERSECTION Example

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40 DIFFERENCE Example

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41 CARTESIAN PRODUCT example

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42 JOIN Example

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43 OUTER JOIN example 1

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44 OUTER JOIN example 2

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45 ADDITIONAL RELATIONAL FUNCTIONS

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46 Aggregate Functions 1. SUM : returns the sum of the values in a specified attribute. 2. COUNT: returns the number of tuples in a specified attribute. 3. AVERAGE: returns the average of the values in a specified attribute. 4. MIN: returns the smallest value in a specified attribute. 5. MAX: returns the largest value in a specified attribute.

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47 Examples NRNameSalaryDepartment 1John100A 5Sara300C 7Tom100A 12AnnienullC EMPLOYEE

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48 1- SUM Find the total of the employees salary R SUM(SALARY) (EMPLOYEE) The result is sum= 500 select sum(salary) from EMPLOYEE Note: Duplicates are not eliminated. Null values are ignored.

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49 2- COUNT Count all the empolyees who get a salary R count(salary) (EMPLOYEE) The result is count=3 select count(salary) from EMPLOYEE

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50 3- AVERAGE Retrieve the average of the employees’ salaries. R AVG( SALARY) (EMPLOYEE) Select avg(salary) from EMPLOYEE

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51 4- MIN Retrieve the minimum salary among all employees. R MIN (SALARY) (EMPLOYEE) The result is min= 100

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52 5- MAX Retrieve the maximum salary among all employees. R MAX( SALARY) (EMPLOYEE) The result is max= 300

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53 Rename Operation Allows us to name, and therefore to refer to, the results of relational-algebra expressions. Allows us to refer to a relation by more than one name. Example: X (E) returns the expression E under the name X If a relational-algebra expression E has arity n, then (A1, A2, …, An) (E) returns the result of expression E under the name X, and with the attributes renamed to A1, A2, …., An.

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54 Rename Operation (DNO,NOEMPS,AVGSAL) DNO COUNT (SSN),AVG( SALARY) (EMPLOYEE) DNO is the grouping attribute

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55 Summary of Relational Algebra Operations Operation Notation Selection Projection Union Difference Intersection Cartesian Product Theta join Equijoin Natural join Outer join Rename Aggregate Function (R) (R) R S R - S R S R X S R F S R * S R S

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