Northern College Diploma Philip Bird Databases Northern College Diploma Philip Bird

What is a Database? Collection of all stored operational data Uses schemas to Separate the data from how its stored Identifies entities and attributes Limit access to approved applications/people Include validation checks Use 4th GL’s to allow easy querying SELECT name, address FROM customer WHERE credit_limit>500

Each entity stores attributes that describes what we know about it Database is composed of tables (entities) linked by key fields (Relational model) Each entity stores attributes that describes what we know about it These can be described by Bachman diagrams Customer (Cust_number, Name, Address, Credit_Limit) 0123 A.Wally 24 High Street 350 0234 S.Jones 12 Huddersfield Rd 600 Customer Order Stock Item Supplier

Advantages Less data duplication New applications are easy to add Easier security monitoring Database can be distributed over many sites

Disadvantages Large and complex systems “All eggs in one basket” Difficult to design (data modelling) Requires management “All eggs in one basket”

Other Issues Security Integrity Consistency and Concurrency Recovery Administration