Relational Modelcse.iitrpr.ac.in/ckn/courses/s2015/w1.pdfSummary • Relational model – Collection of relations or tables – Schema of a relation refers its logical design – Instance

Relational Model

Department of Computer Science and EngineeringIndian Institute of Technology Ropar

Narayanan (CK) Chatapuram Krishnan!

CSL 451 Introduction to Database Systems

Summary•  Relational model

–  Collection of relations or tables–  Schema of a relation refers its logical design–  Instance of a relation refers to its contents at a certain point in time

•  Schema of a relation–  Attributes (atomicity)–  Type of the attributes (domain)–  Constraints on the relation

•  Superkey: a set of one or more attributes whose values are guaranteed to identify tuples in the relation uniquely–  Candidate key – minimal superkey–  Primary key – the chosen candidate key of a relation–  Foreign key – set of attributes in the referencing relation that constitute

the primary key in the referenced relation15/01/15! Department of Computer Science and Engineering !

Indian Institute of Technology Ropar! 2!

Summary•  Schema Diagram–  Pictorial depiction of the schema of a database

•  Relational query languages–  Define a set of operations that operate on

tables(relations), and output tables(relations) as their results

•  Relational Algebra–  An example of the set of operations–  Selection–  Projection–  Natural Join–  Cartesian Product–  Union

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Example of a Schema

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2.1 Consider the following relational databaseemployee (person_name, street, city)

works (person_name, company_name, salary)company (company_name, city)

What are the appropriate primary keys?

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2.2 Consider the foreign key constraint from the dept_name attribute of instructor to the department relation. Give examples of inserts and deletes to these relations, which can cause a violation of the foreign key constraint

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2.3 Consider the time_slot relation. Given that a particular time slot can meet more than once in a week, explain why day and start_time are part of the primary key of this relation, while end_time is not?

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2.4 In the instance of instructor, no two instructors have the same name. From this can we conclude that name can be used as a superkey (or primary key) of instructor?

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2.5 What is the result of first performing the cross product of student and advisor, and then performing a selection operation on the result with the predicate s_id = ID?

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2.6.a Explain in words what the expression does

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�year�2009(takes) ./ student

2.6.b Explain in words what the expression does

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�year�2009(takes ./ student)

2.6.c Explain in words what the expression does

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⇧ID,name,course id

(student ./ takes)

2.7.a Consider the following relational databaseemployee (person_name, street, city)

works (person_name, company_name, salary)company (company_name, city)

Give a relational algebra expression for – finding the names of all employees who live in city “Miami”

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2.7.b Consider the following relational databaseemployee (person_name, street, city)

works (person_name, company_name, salary)company (company_name, city)

Give a relational algebra expression for – finding the names of all employees whose salary is greater than $100,000.

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2.7.c Consider the following relational databaseemployee (person_name, street, city)

works (person_name, company_name, salary)company (company_name, city)

Give a relational algebra expression for – finding the names of all employees who live in “Miami” and whose salary is greater than $100,000.

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2.9 Consider the following bank databasebranch (branch_name, branch_city, assets)

customer (customer_name, customer_street, customer_city)loan(loan_number, branch_name, amount)borrower (customer_name, loan_number)

account (account_number, branch_name, balance)depositor (customer_name, account_number)

a.  What are the appropriate primary keys?b.  Given your choice of primary keys, identify appropriate

foreign keys.

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2.8 Consider the following bank databasebranch (branch_name, branch_city, assets)customer (customer_name, customer_street, customer_city)loan(loan_number, branch_name, amount)borrower (customer_name, loan_number)account (account_number, branch_name, balance)depositor (customer_name, account_number)

Give an expression in relational algebra fora.  Find the names of all branches located in “Chicago”

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2.8 Consider the following bank databasebranch (branch_name, branch_city, assets)customer (customer_name, customer_street, customer_city)loan(loan_number, branch_name, amount)borrower (customer_name, loan_number)account (account_number, branch_name, balance)depositor (customer_name, account_number)

Give an expression in relational algebra forb.  Find the names of all borrowers who have a loan in branch

“Downtown”

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2.13 Consider the following bank databasebranch (branch_name, branch_city, assets)customer (customer_name, customer_street, customer_city)loan(loan_number, branch_name, amount)borrower (customer_name, loan_number)account (account_number, branch_name, balance)depositor (customer_name, account_number)

Give an expression in relational algebra fora.  Find all loan numbers with a loan value greater than $10,000

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2.13 Consider the following bank databasebranch (branch_name, branch_city, assets)customer (customer_name, customer_street, customer_city)loan(loan_number, branch_name, amount)borrower (customer_name, loan_number)account (account_number, branch_name, balance)depositor (customer_name, account_number)

Give an expression in relational algebra forb.  Find the names of all depositors who have an account with a

value greater than $6,000

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2.13 Consider the following bank databasebranch (branch_name, branch_city, assets)customer (customer_name, customer_street, customer_city)loan(loan_number, branch_name, amount)borrower (customer_name, loan_number)account (account_number, branch_name, balance)depositor (customer_name, account_number)

Give an expression in relational algebra forc.  Find the names of all depositors who have an account with a

values greater than $6,000 at the “Uptown” branch.

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2.10 Consider the advisor relation, with s_id as the primary key of advisor. Suppose a student can have more than one advisor. Then, would s_id still be a primary key of the advisor relation? If not, what should the primary key of advisor be?

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2.11 Describe the differences in the meaning between the terms relation and relation schema

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2.14 List two reasons why null values might be introduced into the database

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