2 Chapter 2 The Relational Database Model. 2 2 Hachim Haddouti and Rob & Coronel, Ch2 In this...

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Chapter 2

The Relational Database Model

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Hachim Haddouti and Rob & Coronel, Ch2

In this chapter, you will learn:

• That the relational database model takes a logical view of data

• That the relational database model’s basic components are entities and their attributes, and relationships among entities

• How entities and their attributes are organized into tables

• About relational database operators, the data dictionary, and the system catalog

• How data redundancy is handled in the relational database model

• Why indexing is important

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Logical View of Data

• Relational Database – Designer focuses on logical representation rather

than physical

– Use of table advantageous• Structural and data independence• Related records stored in independent tables• Logical simplicity

– Allows for more effective design strategies

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Logical View of Data (con’t.)

• Entities and Attributes – Entity is a person, place, event, or thing about which

data is collected– Attributes are characteristics of the entity

• Tables– Holds related entities or entity set– Also called relations– Comprised of rows and columns

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Table Characteristics

• Two-dimensional structure with rows and columns

• Rows (tuples) represent single entity

• Columns represent attributes

• Row/column intersection represents single value

• Tables must have an attribute to uniquely identify each row

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Table Characteristics (con’t.)

• Column values all have same data format

• Each column has range of values called attribute domain

• Order of the rows and columns is immaterial to the DBMS

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Keys

• One or more attributes that determine other attributes– Key attribute– Composite key

• Functional dependence ( knowing STU_NUM you can determine STU_LNAME)

• Entity integrity– Uniqueness– No ‘null’ value in key ( NULL= 0?)

• Example: AUI ID#, Invoice#, CIN#

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Keys (con’t.)• Superkey

– Uniquely identifies each entity in a table

• Candidate key– Minimal superkey

• Primary key– Candidate key to uniquely identify all other

attributes in a given row

• Secondary key– Used only for data retrieval

• Foreign key (logical pointer)– Values must match primary key in another table

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Keys (con’t.)

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Integrity Rules

• Entity integrity– Ensures all entities are unique– Each entity has unique primary key (not null)

• Referential integrity– Foreign key must have null value or match primary key

values (every non-null FK must reference an existing primary key)

– Makes it impossible to delete row whose primary key has mandatory matching foreign key values in another table

– No dangling references ( Links in HTML?)

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Relational Database Operators

• Relational algebra determines

table manipulations• Basic operators

– SELECT

– PROJECT

– JOIN

• Other operators– INTERSECT

– UNION

– DIFFERENCE

– PRODUCT

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UnionCombines all rows

• Set-oriented union, duplicates will be eliminated

• (columns and domains must be identical), same Attribute characteristics

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Yields rows that appear in both tables

Intersect

Figure 2.6

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Yields rows not found in other tables

Difference

Figure 2.7

• Set-oriented difference

• columns and domains must be identical, same Attribute characteristics

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Yields all possible pairs from two tablesProduct (Cartesian product)

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Yields a subset of rows based on specified criterion

Select

Figure 2.9

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Yields all values for selected attributes(vertical subset of a table)

Project

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Information from two or more tables is combined by equal AGENT_CODE

Join

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• Links tables by selecting rows with common values in common attribute(s)

• Three-stage process– Product creates one table

– Select yields appropriate rows

– Project yields single copy of each attribute to eliminate duplicate columns

Natural Join Process

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Other Joins• EquiJOIN

– Links tables based on equality condition that compares specified columns of tables

– Does not eliminate duplicate columns – Join criteria must be explicitly defined

• Theta JOIN– EquiJOIN that compares specified columns of

each table using operator other than equality one• Outer JOIN

– Matched pairs are retained – Unmatched values in other tables left null– Two types: right and left

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Data Dictionary and System Catalog

• Data dictionary– Provides detailed account of all tables found within

database– Metadata– Attribute names and characteristics

• System catalog– Detailed data dictionary– System-created database – Stores database characteristics and contents– Tables can be queried just like any other tables– Automatically produces database documentation

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• Points to location• Makes retrieval of data faster

Indexes

Figure 2.31

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Example 1

Identify and describe the components of the database table shown below using correct terminology.

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Example 1 cont.

1 entity set: EMPLOYEE. 5 attributes: EMP_NUM, EMP_LNAME, etc. 10 entities: the workers Friedman, Olansky, Fontein, and

Cruazona. one primary key: the attribute EMP_NUM because it

identifies each row uniquely.• two foreign keys: the attribute DEPT_CODE, which

probably references a department to which the employee is assigned and the attribute JOB_CODE which probably references another table in which you would find the description of the job and perhaps additional information pertaining to the job.

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Example1 cont.

• Cutomer_no is superkey, (Customer_no, Customer_Name) is also superkey for the entity set CUSTOMER

• The combination of primary keys of the participating entity sets forms a super key of a relationship set.

– (customer-id, account-number) is the super key of depositor Concept of superkey is not sufficient

PS: depositor (customer-name, account-number)

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Example 2: Banking

branch (branch-name, branch-city, assets)

customer (customer-name, customer-street, customer-only)

account (account-number, branch-name, balance)

loan (loan-number, branch-name, amount)

depositor (customer-name, account-number)

borrower (customer-name, loan-number)

Stands for PROJECT, for SELECT, & for Union &

INTERSECTION

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Example 2 Queries• Find all loans of over DH1200

Find the loan number for each loan of an amount greater than DH1200

amount > 1200 (loan)

loan-number (amount > 1200 (loan))

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Example 2 Queries• Find the names of all customers who have a loan,

an account, or both, from the bank

Find the names of all customers who have a loan and an

account at bank.

customer-name (borrower) customer-name (depositor)

customer-name (borrower) customer-name (depositor)

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Example 2 Queries• Find the names of all customers who have a loan at the

Ifrane branch of an amount greater than DH1200.

customer-name ( amount > 1200

(borrower.loan-number = loan.loan-number(borrower x loan)))

• Find the names of all customers who have a loan at the branch IFRANE.

customer-name (branch-name=“IFRANE”

(borrower.loan-number = loan.loan-number(borrower x loan)))

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