Databases

Not All Tables Are Created Equal

• Spreadsheets use tables to storedata and formulas associated withthat data

• The “meaning” of data is implicit• Row and column labels are essentially

for decoration, they are not actuallylinked to the data

• Spreadsheet tables lack the necessarystructure to relate entries in one tableto entries in another

Databases

• Structured collections of data• Indexed and organized• In contrast to

– Files (unstructured)– Spreadsheets

(structure but no semantic (meaningful) indexing)

Relational Databases

• Specialized tables that describe relationshipsbetween data items

• Relate the contents of one or more tables• Based on

– Properties that are common between tables– Items that are in common

• This “structure” allows us to filter and composedata efficiently

Logical Data Model

• Entities- tables of anything that can be identified by some number of characteristics, called attributes.

• Attributes have names and values, and these “values” are what is actually stored in a database’s table.

• Relationships are defined between entities (tables)

Database Properties

• Instances are unordered. There is no inherentorder to the rows and columns of a table

• Uniqueness. Each instance (row) must be unique• Keys. Some subset of attributes (columns)

distinguishes each instance in the table. Theremay be several distinct subsets that distinguishinstances; each is a candidate key. Becausedatabases usually have several candidate keys,we choose one and call it the primary key.

Entity Relationship Diagram

• Relationships between entities (tables) can be1-to-many (A department has many faculty)

• There is usually a table for each entity with a1-to-many relationship

Relationships

• One-to-one: There is one instructor per class• One-to-many: A department has many faculty

members• Many-to-many: Each student takes several

classes, and each class has a list of students

A Database’s Schema

• The combination of table attributes and relations specify the structure of a database

• This structure is called the database’s schema• A schema can be specified for an empty

database

Database Operations

• Select - extracts rows from one table to create a new table. Select <Test> From <Table>

Database Operations

• Project - extracts columns from one table tocreate a new table. Select <Fields> From <Table>

Database Operations

• Database operations can be composed

Database Operations

• Product for Combining tables

Product Result

Using Database Products

• Usually table products would be combined with a series of Select and Project operations

• Most often, products would combine tables that do not share common attributes

One More Database Operation

• Join: also combines two tables, but only where the specified attributes match

• Result = Join Nations, NationStats OnNations.Name = NationStats.Name

Database Operations

• Database operations can be composed

Summary

• Databases are specialized tables with names andattributes for all entities

• In a relational database, there exist relationsbetween tables

• The combination of a tables attributes andrelations between the tables in a datbasecomprise it’s schema

• Databases provide basic operations including-Select, Project, Product, Join