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Database Terminology
Introduction
Problems
Terminology
Method Scope Logical Design Technical Optimisation Implementation End

Column

Column One column (data element) contains data of one and the same kind, for example the column postcode.

Compound Key

A compound key (composite key) is a key that consists of multiple columns, because one column is not sufficiently unique.

Example of a compound key: a Dutch postcode & house number jointly identify an address in the Netherlands.

Database

Database A database is a collection of tables, with related data.

Foreign Key

Foreign Key A foreign key is the linking pin between two tables.

Given a value from a row in one table you can access another table to find the right row with related data.

So, one table gives the key for the other, 'foreign' table.

High Frequency Function

A high frequency function is a function that is used very often, hundreds or even thousands times per day.

Index

Index An index in a database resembles an index at the back of a book.

Using a keyword you can find the right page number of rows, within a table.

Join

With a join you combine columns from different tables to one big temporary result table.

For instance, make an overview of readers, with the name of the newspapers that they are subscribed to.

Key

Key A key consists of one or more columns.

If you know the right key value you can find the right row with the desired information. The key is like a real world key, that gives you access to the right row.

A key is unique. A key value can not occur twice in one table. With a key you can find at most one row.

Many to Many Relationship

Many to many relations are everywhere. Unclear Many to Many One newspaper has many readers for example, and one reader reads many newspapers.

Be aware A many-to-many relation is unclear. Many-to-many relations often are a sign that further analysis is required.

Most of the time the relation can be made a lot clearer, by adding a link table, at the 'top' or the 'bottom'.

Many to Many with link at the top For a free house-to-house magazine the link to the reader will be the area, at the top. Many to Many with link at teh bottom

For a paid newspaper, the link is the subsription, at the bottom.

Many to One Relationship

Many to One A Many to One relation is the same as one-to-many, but from a different viewpoint.
  • Many readers live in one area.
  • Many subscriptions can be of one and the same reader.
  • Many subscriptions are for one and the same newspaper.

One to Many Relationship

One to Many Most relations between tables are one-to-many.

Example:

  • One area can be the habitat of many readers.
  • One reader can have many subscriptions.
  • One newspaper can have many subscriptions.

One to One Relationship

One to One A one-to-one relation is exception in databases.

It can occur, but it's often a sign that the database design has opportunities for improvement.

RDBMS

A Relationshipal DataBase Management System is software that

Redundancy

Storing data twice, redundantly to make the system faster.

Which data to store twice is a subject in the chapter Denormalise.

Referential Integrity

Referential Integrity makes sure that a foreign key value always points to an existing row. A broken foreign key

A "dead" link as shown in the picture can not happen anymore.

A number of rules can enforce referential integrity.

  • When creating or updating a row the system checks whether the foreign keys have valid values.
In addition the database designer chooses an action for delete:
  • It's only possible to delete a row in the one-table when there a no more related many-rows.
  • When deleting a row the RDBMS automatically deletes the related data in the many table. This is called a cascaded delete.
  • When deleting the last 'many' the RDBMS automatically deletes the related 'one' row.

Row

Row A row (= tuple, entry) is a group of related data, for example the data of one subscription.

One row can not contain lists.
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Table

Web site Company
www.sum-it.nl SUMit
www.johanvandenboom.nl Johan van den Boom
www.lankelma.nl Lankelma
A table is a matrix with data. A table in a database looks like a simple spreadsheet.

The example above has 3 rows and 2 columns.

Looking for other database terms?

Have a look at the following web sites:
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