Unit 11 - Databases

Attributes

All the different fields in a table for example, firstname, surname, age, address.

Primary key

every value stored in a table has a unique identifier

Entity

category of object/person/event which data needs to be recorded.

Entity Relationship Diagram (ERD)

Shows how each table is linked up by attributes and shows what type of relationship it has between them.

Example of One

to

Example of One

to

Example of Many

to

Composite primary key

2 or more attributes are needed to uniquely define a record. E.g. : EventTicket (EventID, SeatID)

Relations

tables in a relational database. A relation has rows, each row containing one record.

Relational Database

Organised tables that are stored in rows and columns to make it easy to search through.

SQL

A declarative language used for querying/updating tables in a relational database.

1st Normal form

“A database is in 1nf it has no repeating / grouped attributes”.

2nd Normal form

“Already in 1nf, where all its non key attributes (columns) are dependent on the primary key”.

3rd Normal form

“Already in 2nf and contains no attributes that are also dependent on another attribute in a table”

A client server database

provides simultaneous access to the database for several clients.

Database Management System (DBMS)

software on server processes requests for clients, which are individual workstations.

Transaction

A single operation is performed on a database

Records

The rows in tables, storing information.

Foreign key

A field in one table that is linked to another primary key in another table.

Record locking

This prevents simultaneous access to the objects in a database to stop more than 1 person at a time from accessing it.

Serialisation

Transactions are processed one after the other in a queue. Which prevents 2 transactions from overlapping and prevents lost updates.

Commitment ordering

This method prevents data from being lost when 2 users are updating a record simultaneously

How does Commitment Ordering order transactions?

Transactions are ordered by time.Transactions are based on the commit time (when they are completed) rather than when the transactions begun.

How does Seralisation order transactions?

The first transaction that started has to finish before the next transaction can be done.