Concepts of Relational Databases

Fundamental Concepts

Relational data model– A data model representing data in the form of tables

Relations– A 2-dimensional table containing rows and columns of data

Primary Key:

- an attribute (or combination of attributes) that uniquely identifies each row in a relation

Composite Key

- a primary key consisting of more than one attribute

Fundamental Concepts

Foreign Key

- an attribute in a table of a database that serves as the primary key of another table in the same database

Referential integrity constraint

- A rule that states that either each foreign key value must match a primary key value in the other table

An Entity Relationship Diagram for Construction Company

SKILL

Check #Date

BUILDING

Bldg-IDAddr

Type

Q-LevelWORKER

Worker-ID

Name

Hourly Rate

WORKER

Supervises *1 Status *

Has-Skill

1

* Is assigned to *

Transformation to Tables

Worker (Worker-ID, Name, Hourly-Rate, Skill-Type, Supv-ID)

Assignment (Worker-ID, Bldg-ID, Start-Date, Num-Days)

Building (Bldg-ID, Bldg-Address, Type, Q-Level, Status)

Skill (Skill-Type, Bonus-Rate, Hours-Per-Week)

Worker Table

Worker-ID Name Hourly-Rate Skill-Type Supv-ID1235 M. Faraday 12.50 Electric 13111412 C. Nemo 13.75 Plumbing 15202920 R. Garret 10.00 Roofing3231 P. Mason 17.40 Framing1520 H. Rickover 11.75 Plumbing1311 C. Coulomb 15.50 Electric3001 J. Barrister 8.20 Framing 3231

Rows or tuples

Attributes

Sample Tables in Construction Company

Worker-ID Bldg-ID Start-Date Num-Days1235 312 10/10 51412 312 10/01 101235 515 10/17 221412 460 12/08 181412 435 10/15 151412 515 11/05 81311 435 10/08 12

Bldg-ID Bldg-Addr Type Q-Level Status312 123 Elm Office 2 2435 456 Maple Retail 1 1515 789 Oak Resid 3 1210 1011 Birch Office 3 1111 1213 Aspen Office 4 1460 1415 Beech Whse 3 3

Skill-Type Bonus-Rate Hrs-Per-

WeekPlumbing 3.00 35Electric 3.50 37Roofing 2.00 40Framing 5.00 35

Assignment Relation

Skill RelationBuilding Relation

Issues in Database Design

Null values– Not blank or 0; it is simply unknown or inapplicable and may be

supplied at a later time– All workers do not have a supervisor

Functional dependence– If you know a value for A, you know a value for B– A --> B

- Functional Dependency: The value of one attribute (the determinant) determines the value of another attribute.

• Candidate Key: Each non-key field is functionally dependent on every candidate key.

Relational Definitions

Relation Every table has a unique name. Every row is unique. Attributes in tables have unique names. The order of the columns is irrelevant. The order of the rows is irrelevant.

EmployeeID EmployeeName1234 Mary1235 Marvin1223 Molly1220 Melbourne1233 Mallory1324 Michael1332 Maureen

EMPLOYEE(EmployeeID, EmployeeName)

Integrity Constraints

Domain Constraints– Allowable values for an attribute. However you set up the

properties. Entity Integrity

– No primary key attribute may be null. Operational Constraints

– Business rules.

• Referential integrity

– The value of a non-null foreign key must be an actual key value in some relation

Normalization

Normalization is typically a refinement process after the initial exercise of identifying the data objects that should be in the database, identifying their relationships, and defining the tables required and the columns within each table.

Data Normalization

Primarily a tool to validate and improve a logical design so that it satisfies certain constraints that avoid unnecessary duplication of data.

Normalization is the process of organizing it into tables in such a way that the results of using the database are always unambiguous and as intended.

The process of breaking down tables with anomalies to produce smaller, well-structured tables.

Normalization

First Normal Form (1NF) Second Normal Form (2NF) Third Normal Form (3NF)

Steps in normalization

First Normal Form (1NF)

First normal form (1NF). This is the "basic" level of normalization and generally corresponds to the definition of any database, namely:

It contains two-dimensional tables with rows and columns. Each column corresponds to a sub-object or an attribute of the

object represented by the entire table. Each row represents a unique instance of that sub-object or

attribute and must be different in some way from any other row (that is, no duplicate rows are possible).

No multi-valued attributes. All entries in any column must be of the same kind. For example,

in the column labeled "Customer," only customer names or numbers are permitted

First Normal Form (1NF)

Contains no repeating valuesWorker-ID Name Skill-Type Supv-ID Bldg-ID1235 M. Faraday Electric 1311 {312, 515}1412 C. Nemo Plumbing {312, 460,

435, 515}1311 C. Coulomb Electric 435

•If you know the Primary Key, you know valuesfor all other attributes

Second normal form (2NF)

1NF and every non-key attribute is fully functionally dependent on the primary key.

If you have 1 primary key, you are automatically in 2NF!

Every non-key attribute must be defined by the entire key, not by only part of the key.

No partial functional dependencies.If you have 2 fields that make up a composite key – all other fields in

the table must be dependent on both keys in the composite key!

Second Normal Form (2NF)

For example, in a table with three columns containing:

CUSTOMERID

PRODUCT

PRICE (the price would be a function of the customer ID (entitled to a discount) and the specific product.

If PRICE was only related to the PRODUCT – this table would not be in 2NF.

Second Normal Form (2NF)

No non-key attribute is dependent on only a part of the Primary Key

– If only 1 PK, then relation is automatically in 2NF

Decomposed into the following relations:– ASSIGNMENT (Worker-ID, Bldg-ID, Start-Date)– WORKER (Worker-ID, Name)

Worker-ID Bldg-ID Start-Date Name1235 312 10/10 M. Faraday1412 312 10/01 C. Nemo1235 515 10/17 M. Faraday1412 460 12/08 C. Nemo1412 435 10/15 C. Nemo

Third normal form (3NF)

2NF and no transitive dependencies (functional dependency between non-key attributes.)

You don’t want any of your attributes to be dependent on other attributes outside of the primary key!

At the second normal form, modifications are still possible because a change to one row in a table may affect data that refers to this information from another table.

Third Normal Form

For example:Customer Item purchased Purchase price Thomas Shirt $40 Maria Tennis shoes $35 Evelyn Shirt $40 Pajaro Trousers $25removing a row describing a customer purchase (because of a

return perhaps) will also remove the fact that the product has a certain price.

Also, if you change the Purchase price of the Shirt, it will have to be changed multiple times within the table

Third Normal Form

Normalizing the data would mean understanding this and solving the problem by:

dividing this table into two tables, one with information about each customer:

CUSTOMER (CustID, CustName, Address)PRODUCT (Item, Purchase Price, CustID)

Making additions or deletions to either table would not affect the other.

Third Normal (3NF)

Every determinant (left side of a functional dependency) is a key

Decomposed into:– SKILL (Worker-ID, Skill-Type)– BONUS (Skill-Type, Bonus-Rate)

Worker-ID Skill-Type Bonus-Rate

1235 Electric 3.501412 Plumbing 3.001311 Electric 3.50

Paraphrased from Kent (1983)

Each non-key attribute in a relation is dependent on the primary key (1NF), the whole primary key (2NF), and nothing but the primary key (3NF).

Relation with transitive dependency

(a) SALES relation with simple data

(b) Transitive dependency in SALES relation

Removing a transitive dependency

(a) Decomposing the SALES relation

(b) Relations in 3NF

Goals of Normalization

Reduce data redundancy

Improve “modify” activities:– create, update, delete, but not read

Price you pay for Normalization

degraded query, display, reporting Normalization may have the effect of duplicating data

within the database and often results in the creation of additional tables.

(While normalization tends to increase the duplication of data, it does not introduce redundancy, which is unnecessary duplication.)

Steps in Normalization

1NF: a table, without multivalued attributes– if not, then decompose

2NF: 1NF and every non-key attribute is fully functionally dependent on the primary key

– if not, then decompose 3NF: 2NF and no transitive dependencies

– if not, then decompose GENERAL:

– Each table should describe a single theme– Modification anomalies are minimized