Seminar:
Introduction to relational databases
Introduction to the database field:
 The Relational Model
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The Relational Model
A sound theoretical data model
(Codd, 1970).
Based on the mathematical
theory of relations, sets and
first order predicate logic.
De facto standard since the late
eighties.
Many-many implementations –
most SQL-based.
The Notorious Supplier-Part Database (Date)
For instance:
Oracle
MySQL
MS SQL Server
PostgreSQL
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The Relational Model: Concepts
The Notorious Supplier-Part Database (Date)
Central concepts:
Tables (relations).
Columns (attributes).
Type (domain).
Rows (tuples).
Tuples are
unordered.
Tuples are unique.
A relation is a set
(mathematical) of
tuples.
Primary and foreign
keys
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The Relational Model
Data is organised in a number of tables (relations).
Each table has a number (>=1) columns (attributes).
Attributes are atomic and defined over some domain.
A table holds a number (maybe none) rows (tuples).
Tuples are unordered.
Tuples are unique (existence of a key is guaranteed).
A relation is a set (mathematical) of tuples.
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Attributes and Domains
A domain defines the valid value of an attribute.
Domains are based on the built-in standard data types
(int, char etc.) offered by the DBMS.
Theoretically it should be possible to define problem
specific domains as account numbers, IP addresses etc.
and complex aggregate (structured) domain as maps,
diagrams, pictures, sound bites, video clips etc.
More attributes may be defined over the same domain.
An attribute may have the value “empty” (not known
/not defined for this instance). Empty is notated NULL.
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Properties of a Relation
Follows from the fact that relations are (mathematically) sets:
Tuples must be unique within a relation (hence a primary key always exists)
Tuples are unordered (vertically)
Attributes are unordered (horizontally)
Attribute values are atomic
Note the difference to the usual notion of a table
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Keys
A key is a combination of (one or more)
attributes that is:
Unique and
Minimal
An attribute combination that is unique,
but not minimal is called a superkey
The set of all attributes will always be a
superkey, hence a superkey (and a key)
always exists.
A relation (table) may have several
candidate keys.
One these is appointed primary key.
Any primary keys here?
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Associations Between Relations
Is represented by foreign keys.
A foreign key is an attribute
(combination) that corresponds to an
attribute (combination) of the primary
key of some other relation.
A foreign key references a tuple in
another relation and indicates that here
is more information about the entity.
Foreign key attributes and
corresponding primary key attributes
must be defined over compatible
domains (or even the same domain).
Any foreign keys here?
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Integrity Constraints
Domain constraints
Attributes may only hold valid values
Entity Integrity
Primary key attributes may not hold NULL-values
Referential Integrity (foreign key constraint)
A foreign key must either be NULL or reference an
existing primary key in the other relation
Semantic Integrity
Constraints depending on the problem domain
Any constraints here?
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Example: MiniBank
Two tables:
Customers
Accounts
Associated:
An account belongs
to one customer
Association
Any constraints here?
(primary keys – foreign keys)
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Example: MiniBank
What happens if:
We try to insert a customer
with an existing custNo?
We try to insert an account
with a not existing custNo?
Let’s try in MS SQL Server
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Example: MiniBank
Table definitions
(schemas):
Constraint
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Quering a relational database
Database Languages:
Data Definition DDL
Should provide constructs for defining all the previous
(as “create table)
Data Manipulation DML (queries, insert, delete, update)
procedural (How?)
nonprocedural (What?)
The Relational Algebra is a procedural DML
SQL includes a (sort of) nonprocedural DML
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The Relational Algebra
Data Manipulation in the Relational Model
Operates on relations, which are input to the
operations is tables and the result is a table
Operations
Row selection (RESTRICT/SELECT)
Column selection (PROJECT)
Combining tables (JOIN)
Set operations (UNION, INTERSECTION, DIFFERENCE,
PRODUCT)
More advanced operations (OUTER (LEFT/RIGTH) JOIN)
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Relational Algebra - Overview
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Example: MiniBank
Retrieve information about
customer number 3:
Row selection on custNo = 3 from
Customer
Retrieve account number,
balance and customer number
for accounts with a balance
between 1000 and 2000:
Row selection on 1000 <= balance
and balance <= 2000 from Account
Column selection on accNo,
balance, custNo
Retrieve information about
customer Tommy and his
accounts:
Row selection on name = ‘Tommy’
from Customer
Join with Account on custNo
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