Database Technologies. Madalina CROITORU IUT Montpellier

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1 Database Technologies Madalina CROITORU IUT Montpellier

2 Part 2 RELATIONAL ALGEBRA

3 Background notions I A database relation is a set. Sets can be refined: One can select a subset of elements based on specifically desired elements. σ selection predicate (relation) One can project a subset based on needed attributes of all the elements of the relation π a1,a2,, an (relation)

4 Background notions II Sets can be manipulated with classical set operations : union, intersection, difference. Let us discuss each: Union: The union of two relations R and S defined a relation that contains all the tuples of R, or S, or both R and S, duplicated tuples being removed. R and S must be union compatible. Union is possible only if the schemas of the two relations match: if they have the same number of attributes with each pair of the corresponding attributes having the sae domain. Sometimes projection can be used to make two relations match. Set difference between R and S defines a relation consisting of the tuples that are in R but not in S. The R and S must match. Intersection between R and S defines a relation consisting of the tuple that are both in R and S. The R and S must match. Cartesian product: defines a relation that is the concatenation of every tuple of relation R with every tuple of relation S. If one relation has I tuples and N attributes and the other has J tuples and M attributes then the cartesian product relation will contain I x J tuples with N + M attributes.

5 Examples

6 Examples I List all staff with a salary greater than (selection) Produce a list of salaries for all staff showing only the staffno, fname, lname ad salary details (projection) List all cities where there is either a branch office or a property for rent (union). Note: to produce matching relations we first use the projection to project Branch and PropertyforRent over the attribute city, eliminating duplicates when necessary. List all cities where there is a branch office but no properties to rent (set difference). Note: we also produce matching relations by projecting over city. List all cities where there is both a brach office and at least one property for rent (intersection).

7 Examples II List all names and comments of all clients who have viewed a property for rent (cartesian product of the projection over clientno, fname,lname of Client and clientno, proertyno and comment of Viewing followed by a selection on those tuples where the Client.clientNo=Viewing.clientNo). One can use intermediate names (views) for the relations obtained at each step: TempViewing(clientNo, propertyno, comment); TempClient(clientNo, fname, lname); Comment (clientno, fname, lname, vclientno, proeprtyno, comment) and, finally; Result.

9 Exercise 1 Use the hotel schema from last course and write the relational algebra expressions for the following queries: List all hotels List all single rooms with prices below 20 per night List the names and cities of all guests List the price and type of all rooms at the Royal Hotel List all guests currently staying at the Royal Hotel List the details of all rooms at the Royal Hotel including the name of the guest staying in the room, if the room is occupied List the guest details (guestno, guestname and guest Address) of all guests staying at the Royal Hotel

10 Exercise 2 The following tables form part of a database: Employee(empNo, fname, lname, address, DOB, sex, position, deptno) Department(deptNo, deptname, mgrempno) Project(projNo, projname, deptno) WorksOn(empNo, projno, dateworked, hoursworked) Write the following queries in relational algebra: List all employees List all the details of the employees that are female and born after 1990 List all employees who are not managers and are paid more than 1500 Produce a list of the names of all employees who work on the SCCS project Produce a complete list of managers who are due to retire this year Find out how many managers are female Produce a report of all projects under the IT department Using the union operator retrieve the list of employees who are neither managers nor supervisors. Attributes to be retrieved are first name, last name, position, sex and department number

CMP-3440 Database Systems

CMP-3440 Database Systems Relational DB Languages Relational Algebra, Calculus, SQL Lecture 05 zain 1 Introduction Relational algebra & relational calculus are formal languages associated with the relational