Riyadh Philanthropic Society For Science Prince Sultan College For Woman Dept. of Computer & Information Sciences CS 340 Introduction to Database Systems Chapter 6: The Relational Algebra and Relational Calculus

1 Outline Introduction Relational Algebra Unary Relational Operations Relational Algebra Operations from Set Theory Binary Relational Operations Additional relational Operations Examples of Queries in Relational Algebra Relational Calculus Introduction to Tuple Relational Calculus Introduction to Domain Relational Calculus

Chapter 6: The Relational Algebra and Relational Calculus2 Introduction A data model must include a set of operations to manipulate relations and produce new relations as answers to queries. Formal languages for the relational model: Relational algebra: specifies a sequence of operations to specify a query. Relational calculus: specifies the result of a query. (without specifying how to produce the query result) Tuple calculus. Domain Calculus.

Chapter 6: The Relational Algebra and Relational Calculus3 Example

Chapter 6: The Relational Algebra and Relational Calculus4 Relational Algebra The basic set of operations for the relational model is known as the relational algebra. These operations enable a user to specify basic retrieval requests. The result of a retrieval is a new relation, which may have been formed from one or more relations. A sequence of relational algebra operations forms a relational algebra expression.

Chapter 6: The Relational Algebra and Relational Calculus5 Relational Algebra Two basic sets of operations: Relational operators (specific for relational databases): Select. Project. Join. Division. Set Theoretic Operators: Union. Intersection. Minus. Cartesian product.

Chapter 6: The Relational Algebra and Relational Calculus6 Basic Relational Algebra Operations SelectProjectUnionIntersectionDifference R S R S R S Cartesian Product *

Chapter 6: The Relational Algebra and Relational Calculus7 Unary Relational Operations - SELECT Selects a subset of the tuples from a relation that satisfy a selection condition. Syntax:  ( ) Examples: Select the EMPLOYEE tuples whose department is 4.  Dno=4 (EMPLOYEE) Select the tuples for all employees who either work in department 4 and make over $25,000 per year, or work in department 5 and make over $30,000.  (Dno=4 AND Salary>25000) OR (Dno=5 AND Salary>30000) (EMPLOYEE)

Chapter 6: The Relational Algebra and Relational Calculus8 Unary Relational Operations - SELECT  (Dno=4 AND Salary>25000) OR (Dno=5 AND Salary>30000) (EMPLOYEE)

Unary Relational Operations - SELECT The SELECT operation  (R) produces a relation S that has the same schema as R The select operation is commutative:  (  (R)) =  (  (R)) A cascaded SELECT operation may be replaced by a single selection with a conjunction of all the conditions:  (  (  (R))) =  AND AND (R) Chapter 6: The Relational Algebra and Relational Calculus9

Unary Relational Operations - PROJECT Selects certain columns from the table and discards the other columns. Syntax:  ( ) Example: list each employee’s first and last name and salary.  Lname, Fname, Salary (EMPLOYEE) The project operation removes any duplicate tuples, so the result of the project operation is a set of tuples and hence a valid relation. 10

Chapter 6: The Relational Algebra and Relational Calculus11 Unary Relational Operations - PROJECT  Lname, Fname, Salary (EMPLOYEE)

Chapter 6: The Relational Algebra and Relational Calculus Sequence of Operations & RENAME Operation We can: Nest the relational algebra operations as a single expression, or Apply one operation at a time and create intermediate result relations, and give it a name. Example: retrieve the first name, last name, and salary of all employees who work in department number 5.  Fname, Lname, Salary (  Dno=5 (EMPLOYEE)) or TEMP  Dno=5 (EMPLOYEE) R  Fname, Lname, Salary (TEMP) The rename operator is . 12

Chapter 6: The Relational Algebra and Relational Calculus Sequence of Operations & RENAME Operation  Fname, Lname, Salary (  Dno=5 (EMPLOYEE)) TEMP  Dno=5 (EMPLOYEE) R(First_name,Last_name,Salary)  Fname, Lname, Salary (TEMP) 13

Chapter 6: The Relational Algebra and Relational Calculus Relational Algebra Operations From Set Theory UNION, INTERSECTION, & MINUS Operands need to be union compatible for the result to be a valid relation. In practice, it is rare that two relations are union compatible (occurs most often in derived relations). 14

Chapter 6: The Relational Algebra and Relational Calculus Relational Algebra Operations From Set Theory - UNION The result of this operation, denoted by R  S, is a relation that includes all tuples that are either in R or in S or in both R and S. Duplicate tuples are eliminated. 15

Chapter 6: The Relational Algebra and Relational Calculus16 Relational Algebra Operations From Set Theory - UNION Example: retrieve the SSN of all employees who either work in department 5 or directly supervise an employee who works in department 5. DEP5_EMPS  Dno=5 (EMPLOYEE) RESULT1  Ssn (DEP5_EMPS) RESULT2(Ssn)  Super_ssn (DEP5_EMPS) RESULT RESULT1  RESULT2

Chapter 6: The Relational Algebra and Relational Calculus17 Relational Algebra Operations From Set Theory - INTERSECTION The result of this operation, denoted by R S, is a relation that includes all tuples that are in both R and S. 

Chapter 6: The Relational Algebra and Relational Calculus18 Relational Algebra Operations From Set Theory - MINUS The result of this operation, denoted by R - S, is a relation that includes all tuples that are in R but not in S.

Chapter 6: The Relational Algebra and Relational Calculus The Set Operations b. STUDENT  INSTRUCTOR c. STUDENT INSTRUCTOR d. STUDENT - INSTRUCTOR e. INSTRUCTOR - STUDENT 19 

Chapter 6: The Relational Algebra and Relational Calculus24 Binary Relational Operations - JOIN The JOIN operation is used to combine related tuples from two relations into single tuples. Syntax: R S (does not require union compatibility of R and S). Example: retrieve the name of the manager of each department. DEPT_MGR DEPARTMENT Mgr_ssn=Ssn EMPLOYEE RESULT  Dname, Lname, Fname (DEPT_MGR)

Chapter 6: The Relational Algebra and Relational Calculus25 Binary Relational Operations - EQUIJOIN Joins conditions with equality comparisons only. In the result of an EQUIJOIN, one or more pairs of attributes always have identical values in every tuple.

Chapter 6: The Relational Algebra and Relational Calculus26 Binary Relational Operations - NATURAL JOIN Because one of each pair of attributes with identical values is superfluous, a new operation called NATUARAL JOIN was created to get rid of the second (superfluous) attribute in an EQUIJOIN condition. The standard definition requires that the two join attributes, or each pair of corresponding join attributes, have the same name in both relations. Syntax: R * S

Binary Relational Operations - NATURAL JOIN To apply a NATURAL JOIN on the DNUMBER attribute of DEPARTMENT and DEPT_LOCATIONS, it is sufficient to write: DEPT_LOCS DEPARTMENT * DEPT_LOCATIONS Chapter 6: The Relational Algebra and Relational Calculus27

Chapter 6: The Relational Algebra and Relational Calculus28 Binary Relational Operations - NATURAL JOIN To apply a NATURAL JOIN on the department number attribute of DEPARTMENT and PROJECT: DEPT  (Dname,Dnum,Mgr_ssn,Mgr_start_date) (DEPARTMENT) PROJ_DEPT PROJECT * DEPT

Chapter 6: The Relational Algebra and Relational Calculus Additional Relational Operations Aggregate Functions and Grouping The first type of request that cannot be expressed in the basic relational algebra is to specify mathematical aggregate functions on collection of values from the database. Common functions applied to collections of numeric values include: SUM. AVERAGE. MAXIMUM. MINIMUM. The COUNT function is used for counting tuples or values. 31

Additional Relational Operations Aggregate Functions and Grouping Another common type of request involves grouping the tuples in a relation by the value of some of their attributes and then applying an aggregate function independently to each group. Syntax: (R) Example: COUNT Ssn, AVERAGE Salary (EMPLOYEE) Chapter 6: The Relational Algebra and Relational Calculus32

Chapter 6: The Relational Algebra and Relational Calculus Additional Relational Operations Aggregate Functions and Grouping Example: Dno COUNT Ssn, AVERAGE Salary (EMPLOYEE) Example:  R (Dno, No_of_employees, Average_sal) (Dno COUNT Ssn, AVERAGE Salary (EMPLOYEE)) 33

Additional Relational Operations OUTER JOIN Operations In NATUARAL JOIN, the following tuples are eliminated from the join result: Tuples without a matching (or related) tuple. Tuples with null in the join attributes. A set of operations, called OUTER JOINs, can be used when we want to keep all the tuples: in R, or in S, or in both relations in the result of the join. Chapter 6: The Relational Algebra and Relational Calculus35

Additional Relational Operations OUTER JOIN Operations The left outer join operation ( R S) keeps every tuple in R, if no matching tuple is found in S, then the attributes of S in the join result are filled with null values. The right outer join operation ( R S) keeps every tuple in S, if no matching tuple is found in R, then the attributes of R in the join result are filled with null values. The full outer join operation (R S) keeps all tuples in both the left and the right relations when no matching tuples are found, padding them with null values as needed. Chapter 6: The Relational Algebra and Relational Calculus36

Examples of Queries in Relational Algebra Query 1: Retrieve the name and address of all employees who work for the ‘Research’ department. Chapter 6: The Relational Algebra and Relational Calculus38 R_DEPT  Dname=‘Research’ (DEPARTMENT) R_EMPS (R_DEPT Dnumber=Dno EMPLOYEE) RESULT  Lname, Fname, Address (R_EMPS)

Chapter 6: The Relational Algebra and Relational Calculus39 Examples of Queries in Relational Algebra Query 2: For every project located in ‘Stafford’, list the project number, the controlling department, and the department manager’s last name, address, and birth date. S_PROJS  Plocation=‘Stafford’ (PROJECT) C_DEPT (S_PROJS Dnum=Dnumber DEPARTMENT) P_DEPT_MGR (C_DEPT Mgr_ssn=Ssn EMPLOYEE) RESULT  Pnumber, Dnum, Lname, Address, Bdate (P_DEPT_MGR)

Chapter 6: The Relational Algebra and Relational Calculus41 Examples of Queries in Relational Algebra Query 4: Make a list of project numbers for projects that involve an employee whose last name is ‘Smith’, either as a worker or as a manager of the department that controls the project. SMITHS(Essn)  Ssn (  Lname=‘Smith’ (EMPLOYEE)) SMITH_W_PROJ  Pno (WORKS_ON * SMITHS) MGRS  Lname, Dnumber (EMPLOYEE Ssn=Mgr_ssn DEPARTMENT) SMITH_M_DEPTS(Dnum)  Dnumber (  Lname=‘Smith’ (MGRS)) SMITH_M_PROJS(Pno)  Pnumber (SMITH_M_DEPTS * PROJECT) RESULT (SMITH_W_PROJS  SMITH_M_PROJS)