1. CompSci 280 S2 2107 Introduction to Software Development
The Relational Model

2. Today’s Agenda Topics: Reading: Introduction The Relational Model
Database Relationships
One to One
One to Many
Many to Many
Constraints
Reading:
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3. Modeling an App with a 3-Tier Architecture
Written docs
UI models (e.g. screen diagrams)
Flow or sequence charts
State machines
Data models (e.g. class diagrams)
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4. 1.Introduction What is a Database Management System DBMS? Definition
Software that manages and stores the data. The applications interact with the system, which retrieves and manipulates the data.
Advantages of Database systems over file systems
Data Integrity;
Controlled Redundancy;
Standards can be proposed since the data definitions are also shared;
The ability to implement decisions on access control;
The ability to balance conflicting needs by tuning and optimising
Security of data.
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5. 2.The Relational Model Was introduced by Codd in 1970 (journal paper);
It is based on a simple and uniform data structure - the relation - and it has a solid theoretical foundation;
The Relational Model represents the Database as a collection of Relations, which are sets from a mathematical point of view;
One Entity type is a Relation;
One Relationship Type is either a Relation or one or more attributes;
The retrieval of information involves manipulating the Relations using set operations and also operators defined in the relational algebra;
Examples of relational database systems: Oracle, Microsoft SQL Server, Sybase SQL Server,
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6. 2.The Relational Model Relational Database
Data from several tables is tied together (related) using a field that the tables have in common
A Simple Relational Database Example
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7. 2.The Relational Model Some Rules …
Every table has a unique name.
Attributes in tables have unique names.
Every row/tuple is unique.
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8. 2.The Relational Model Concept
Relational modeling uses primary keys and foreign keys to maintain relationships
Primary Key (cannot have null value) - attribute(s) that uniquely identify a tuple in a table.
Foreign Key - an attribute that is a primary key in another table.
Composite keys are primary keys that are made of more than one attribute
Weak entities
Associative entities
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9. 2.The Relational Model Customer Table
Mapping UML class diagrams to Relational Data Base Tables
CREATE TABLE Customer (
customerID INT NOT NULL primary key,
name VARCHAR(50) NOT NULL,
tempAmount decimal );
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10. 3.Database Relationships
There are several types of database relationships.
One to One Relationships
One to Many and Many to One Relationships
Many to Many Relationships
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11. 3.Database Relationships One to One Relationships
There are two tables:
Customers table:
Addresses table:
We have a relationship between the Customers table and the Addresses table.
If each address can belong to only one customer, this relationship is "One to One".
Keep in mind that this kind of relationship is not very common.
Normally one table is enough:
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12. 3.Database Relationships One to Many Relationships
This is the most commonly used type of relationship. Consider an e-commerce website, with the following:
Customers can make many orders.
Orders can contain many items.
Items can have descriptions in many languages.
Example:
Each customer may have zero, one or multiple orders. But an order can belong to only one customer.
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13. 3.Database Relationships Many to Many Relationships
In some cases, you may need multiple instances on both sides of the relationship.
For example, each order can contain multiple items. And each item can also be in multiple orders.
We need to create an extra table:
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14. 4.Constraints Primary & Foreign Keys
Tables can be related through primary/foreign key relationships (e.g. A customer may order one or more orders)
Primary key
Guarantees the uniqueness of a row
Can be composed of one or more columns
Foreign key
Establishes logical relationship between tables
One or more columns of a table that match the primary key of another table
CustomerID
FirstName
LastName
ALFKI
Maria Anders
Alfreds Futterkiste
ANATR
Ana Trujillo
Ana Trujillo ...
Foreign Key 1 ∞
Relation
OrderID
CustomerID
OrderDate
10248
WILMK
04-Jul-1996
10249
TRADH
05-Jul-1996
Primary Key
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15. 4.Constraints
A CONSTRAINT is a rule that limits (or restricts) what data can be placed in a column of a table.
Constraints are used to ensure that the data is kept consistent and meaningful.
UNIQUE Constraints
Ensures that every row contains a different value
NOT NULL Constraints
Ensures that there are no nulls in that column
PRIMARY KEY Constraints
Combination of Unique and Not Null constraint
Referential Integrity Constraints
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16. 4.Constraints Referential Integrity Constraints
Main type of data validation within relational database
Ensures relationships are maintained between the data in one table with data in another table
Performed when data is being changed
Performed during Insert, Update and Delete
Statements will fail if Referential Integrity is not maintained
For examples:
Case 1: add a new row to the child table
Case 2: update/delete an existing row
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17. 4.Constraints Case 1 Parent table: customer, child table: order
In the Data Table(child), a value can always be Deleted or set to null, but any new value must pass validation
For example:
Insert a new order for a new customer:
20001, AXETH (Axen Thomas),
Solution:
Add “Axen Thomas” into the Customer table FIRST
Then, add the new order to the Order table
CustomerID
FirstName
LastName
ALFKI
Maria Anders
Alfreds Futterkiste
ANATR
Ana Trujillo
Ana Trujillo ...
Foreign Key
OrderID
CustomerID
OrderDate
10248
WILMK
04-Jul-1996
10249
TRADH
05-Jul-1996
Primary Key
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18. There is an order made by WILMK in the order table
4.Constraints Case 2
In the List of Valid Values in the parent table, a new valid value can always be added, but a value can only be deleted if it is not used at all in the Data Table.
For example:
Remove WILMK from the customer table
Solution:
Use the RI options to “Cascade”
All the ROWS of the Data Table are DELETED if they were using the value that is being deleted
There is an order made by WILMK in the order table
CustomerID
FirstName
LastName
ALFKI
Maria Anders
Alfreds Futterkiste
ANATR
Ana Trujillo
Ana Trujillo ...
Foreign Key
OrderID
CustomerID
OrderDate
10248
WILMK
04-Jul-1996
10249
TRADH
05-Jul-1996
Primary Key
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