Database notes

Databases:

• are persistent organised stores of related data on computer systems
• they are organised so that computer programs can easily select or edit data items
• persistent means that they are stored in secondary stroage so that they can be accessed later
• handled by software
• software has to be able to create, retrieve, update and delete data in a database. (C.R.U.D)
• a serial file is a file of items one after another (takes too long to find a particular item)
• a sequential file is a serial file in order (quicker to find particular items)
• an indexed sequential file is a sequential file that is accessed using an index, which is a seperate file
• a flat file database is a database consisting of only one table
• a field is a characteristic of something stored on a database
• a record is all the data about one item in a database
• an entity is something that we store data about in a database
• an attribute is a characteristic of an entity
• data redundancy is the unnecessary repitition of data
• data integrity is the state of data being as it should be, reflecting reality
• validation is the process of checking data as it is input to ensure it is reasonable
• verification is the process of checking that data is true or correct

Validation checks:

• check digit- a calculation is performed on a number that generates another digit which is appended to the number. When the data is entered, the calculation is repeated and if the same check digit is not generated, the input is rejected
• format check- the data might have to conform to a particular pattern
• length check- the data must be within certain limits
• lookup check- some fields might be looked up to see if they are from an allowed range
• presence check- this is to ensure that all important information has been included
• range check- a number must fall within limits
• type check- the data type might be restricted

Types of database:

• hierarchical- used when items are composed of other items e.g. different sections of a factory
• relational- stores data in tables
• each table contains data about one entity
• a table is called a relation
• a table has attributes arranged as columns, which are identified by their names
• a table has rows which are equivalent to records
• tables must…