databases

entity

an object, person, place, concept, or thing of interest to an organisation about which data is recorded

attribute

a property or characteristic of an entity

primary key

a unique identifier that identifies a record in a table

composite primary key

a primary key made up from two or more other keys

foreign key

a field in one table that is also a primary key in another table

third normal form

no transitive dependencies, every field depends upon the key, the whole key and nothing but the key

update statement

UPDATE table

SET attribute=value

WHERE condition

delete command

DELETE FROM table

WHERE

insert command

INSERT INTO table(attributes)

VALUES (values)

timestamp ordering

timestamps are generated for each transaction. database records timestamps of last read and last write of records. rules are applied to determine if processing a transaction will result in loss of data integrity.

record locks

when a transaction is occurring on a record, the record is locked so that other transactions cannot be made until the record is unlocked, preventing simultaneous transactions

problems with many to many relationship

redundant data, anomalies will exist when deleting data, inconsistencies when updating data, anomalies when inserting data

serialisation

rather than locking a field, requests from other users are placed in a queue and executed sequentially

commitment ordering

when a database uses an algorithm to optimise the order in which transactions are completed to minimise issues from occurring with the database