Fundamentals of Databases Flashcards
Entity
Thing about which data is to be stored
Attributes
Characteristics or other information about entities
Relationship of car and owner
One-to-one
Car ——- Owner
Relationship of car and passenger
Each car has many passengers, each passenger sits in one car
Car ——-<- Passenger
Relationship of car and driver
Each driver can drive many different cars, each car is driven by many drivers
Car ->——<- Driver
Primary Key
Attribute that provides an unique identifier for every entity in the database table
Foreign Key
An attribute in a table which is a primary key in another
Composite Primary Key
When it is not possible to form a primary key from one attribute so they are combined
Database Normalisation
Efficient without any compromise to the integrity of their data. Entities contain no redundant/repeated data
Advantages of Database Normalisation
Faster searching and sorting, easier to maintain and data consistency is improved
First Normal Form
Contains no repeating attributes (atomic)
Second Normal Form
Database must satisfy first normal form and partial key dependencies are removed (databases with composite Primary keys have a non-key attribute doesn’t depend on the whole of the composite key
Third normal form
Conforms to second normal form and must have no non-key dependencies
Concurrent access
When users attempt to access to access the same field at the same time
Record Locks
When a record is being accessed by one user, it is locked to other users until the first user has finished using it
Serialisation
Requests from other users trying to access the field are placed in a queue
Timestamp ordering
When multiple commands are sent to the same field, they are given a timestamp which indicates the time at which the command was initiated
Commitment ordering
Uses an algorithm to work out optimum order to execute commands
