Database elements

RHS – 2009 2

Database elements

• A database can be compared to an old-fashioned file cabinet– The entire canibet is the

database– A drawer is a table– A folder is a record– One piece of information

is a field

RHS – 2009 3

Table

• A table contains data about one specific type of entity, and always on the same form

Person

Name Address Phone Born Married

Per Jensen Skolevej 12 31458791 17-07-1962 No

Jane Høst Allégade 70 52442109 31-03-1971 Yes

Allan Ravn Svinget 49 44553177 12-12-1977 Yes

Ole Olsen Junivej 112 41764430 26-04-1950 Yes

Ditte Dorfmann Dalstrøget 3 43519982 11-08-1972 No

RHS – 2009 4

Table

• A table should have a proper name, reflecting the type of entity stored in the table

• Some tables are directly related to entities, others are related to relations between entities

• A table definition consists of definitions of data fields, each with a name and type

RHS – 2009 5

Record

• A record is a single row in a specific table, containing information about a single entity

Person

Name Address Phone Born Married

Per Jensen Skolevej 12 31458791 17-07-1962 No

Jane Høst Allégade 70 52442109 31-03-1971 Yes

Allan Ravn Svinget 49 44553177 12-12-1977 Yes

Ole Olsen Junivej 112 41764430 26-04-1950 Yes

Ditte Dorfmann Dalstrøget 3 43519982 11-08-1972 No

RHS – 2009 6

Record

• All records in a specific table always has the same form

• All records in a specific table must be unique (have a key field)

• A key field consists of one or more fields in the table definition, the combination of which is always unique

RHS – 2009 7

Record

• What makes a Person unique…?

PersonName Address Phone Born Married

Per Jensen Skolevej 12 31458791 17-07-1962 No

Jane Høst Allégade 70 52442109 31-03-1971 Yes

Allan Ravn Svinget 49 44553177 12-12-1977 Yes

Ole Olsen Junivej 112 41764430 26-04-1950 Yes

Ditte Dorfmann Dalstrøget 3 43519982 11-08-1972 No

RHS – 2009 8

Field

• A field is a single piece of information, about one specific entity in the table

Person

Name Address Phone Born Married

Per Jensen Skolevej 12 31458791 17-07-1962 No

Jane Høst Allégade 70 52442109 31-03-1971 Yes

Allan Ravn Svinget 49 44553177 12-12-1977 Yes

Ole Olsen Junivej 112 41764430 26-04-1950 Yes

Ditte Dorfmann Dalstrøget 3 43519982 11-08-1972 No

RHS – 2009 9

Field

• A field has a name and a type

• The type of the field should reflect the possible content of the field– String (text data)– Number (numeric data)– Date (dates, time)– Yes/No (binary choices)– Binary (picture, attached file,…)

RHS – 2009 10

Designing a database

• Designing a database looks fairly easy…

• Designing it correctly can be more challenging!

• Correctly: Avoid the two arch enemies of databases– Redundancy– Inconsistency

RHS – 2009 11

Designing a database

• Redundancy: the phenomenon that the same data occurs multiple times in the database– Makes the database unnecessarily large– Data processing becomes slower– Increases the risk of inconsistency

RHS – 2009 12

Designing a database

Name Address Phone Born CoursePer Jensen Skolevej 12 31458791 17-07-1962 English

Per Jensen Skolevej 12 31458791 17-07-1962 IT

Ditte Dorfmann Dalstrøget 3 43519982 11-08-1972 English

Ditte Dorfmann Dalstrøget 3 43519982 11-08-1972 Economics

Ditte Dorfmann Dalstrøget 3 43519982 11-08-1972 Math

Student

RHS – 2009 13

Designing a database

Id Name Address Phone Born

1 Per Jensen Skolevej 12 31458791 17-07-1962

2 Ditte Dorfmann Dalstrøget 3 43519982 11-08-1972

Student

Enrollment Id Course

1 English

1 IT

2 English

2 Economics

2 Math

RHS – 2009 14

Designing a database

• Inconsistency: the phenomenon, that some data is in conflict with each other. Not all data can be correct

• Which data should we trust then?

• If the database is free of redundancy, simple inconsistency cannot occur

RHS – 2009 15

Designing a database

Name Address Phone Born CoursePer Jensen Skolevej 12 31458791 17-07-1962 English

Per Jensen Skolevej 21 31458791 17-07-1962 IT

Ditte Dorfmann Dalstrøget 3 43519982 11-08-1972 English

Ditte Dorfmann Dalstrøget 3 43519982 11-08-1972 Economics

Ditte Dorfmann Bakkehøj 22 33128769 28-03-1976 Math

Student

RHS – 2009 16

Designing a database

Exercise1. We wish to construct a database for data about our friends. More

specifically, we would like to store: Name, Address, Zipcode, Phone, E-mail, Marital status, date of last contact, and a picture (feel free to include more data if you wish)

2. A single table called Friend might be enough. Design such a table, including definitions of each data field needed in the table. Remember that the table should contain a key field

3. Look at your final table definition. Is there any danger of redundancy and/or inconsistency?

4. (If yes in 3.) Try to think of a better design, to eliminate the risk of redundancy and/or inconsistency? (Tip: you may need to include more tables in the database)