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Introduction to the database field:

The Relational Model

Seminar:Introduction to relational databases

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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.

FEN 2015-08-31

The Notorious Supplier-Part Database (Date)

For instance:OracleMySQLMS SQL ServerPostgreSQL

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The Relational Model: Concepts

FEN 2015-08-31

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

The Notorious Supplier-Part Database (Date)

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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 andMinimal

An attribute combination that is unique, but not minimal is called a superkeyThe 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.

FEN 2015-08-31

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).

FEN 2015-08-31

Any foreign keys here?

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Integrity Constraints

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Domain constraintsAttributes may only hold valid values

Entity IntegrityPrimary 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 IntegrityConstraints depending on the problem domain

Any constraints here?

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Example: MiniBank

Two tables:CustomersAccounts

Associated:An account belongs to one customer

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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: MiniBankTable definitions (schemas):

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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 DMLSQL includes a (sort of) nonprocedural DML

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The Relational Algebra

Data Manipulation in the Relational ModelOperates on relations, which are input to the operations is tables and the result is a tableOperations

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: MiniBankRetrieve 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 AccountColumn selection on accNo, balance, custNo

Retrieve information about customer Tommy and his accounts:

Row selection on name = ‘Tommy’ from CustomerJoin with Account on custNo

FEN 2015-08-31