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Transcript
Database Features Lecture 2 Desirable features in an information system • • • • • • Integrity Referential integrity Data independence Controlled redundancy Security Privacy Integrity • Valid and consistent stored data – If there is a rule that a schoolchild should be between 4 and 18 years old, is the rule: • • • • 4<age<18 or 4<=age<18 or 4<=age<=18 or 4<age<=18? – By keeping the rule with the data definition, this ensures integrity. Referential integrity • Data has referential integrity if it is valid in its own right and valid in relation to another occurrence. • E.g. a student is not a student unless he/she is studying a course • The course listed against the student must exist. – Master : detail relationship – Implemented through Foreign Keys. Data independence • This is where the data is independent of the program that uses it: – E.g. • Select stock_code from stock; – This will work, regardless of the definition of the table stock, unless • You have no access to the table stock • There is no column ‘stock_code’ in the table stock – This allows us to change • the front layers of software without changing the tables • The tables without changing the front layers (some recompilation may be required) Controlled redundancy • Controlled redundancy – Redundant data causes problems • E.g. staff details – addresses and phone numbers, • If recorded separately for payroll and human resources, then when one is changed, the other is not. • Bank accounts and credit card accounts. – NO redundancy causes problems • One server goes down, there is no other server • In order to retrieve information, a traversal of three tables is required – inefficient. Security • From theft and fraud • Loss of confidentiality • DBMS controls: – Granting of different privileges to different users • You will have SELECT access to the tables in the BUILDER schema. • You have owner access to your own Schema and to your team schema. • You may grant and revoke access to your tables to others in your class. – – – – Backup and Recovery Journalling Commit and Rollback Encryption Privacy • Can be maintained through: – Use of views • E.g. although the staff table shows full details of – Salary – Direct payments to family members – Loan repayments, etc. • The person in charge of the staff roster only sees: • Name, rank, qualifications, free slots (probably a different table) Database definition • a computerised record-keeping system • used by a range of users who have different requirements – minimal enquiries – in-depth updating – restructuring • A well-implemented database will have data integrity, data independence, controlled redundancy, security and privacy, for all users. Uses of a Database • Generally used for on-line transaction processing (OLTP) • Data Warehouses are a hybrid of databases which are used for On-line analytical processing (OLAP) File systems • Predecessors of Database Systems • File types: – Sequential or serial • Suitable for use on a serial medium – e.g. a tape. • Access is determined by the ‘position’ of the reader on the serial file or tape. No sharing. – Indexed sequential • Closest in form to relational databases • Can be used to implement relational databases, but all of the DBMS features must be programmed in. – Relative • Based on the offset of the record from the start of the file. • Field values are not involved in keys. File systems • Sequential or serial • Indexed sequential • Relative Database definition • a computerised record-keeping system • used by a range of users who have different requirements – minimal enquiries – in-depth updating – restructuring • A well-implemented database will have data integrity, data independence, controlled redundancy, security and privacy, for all users. Uses of a Database • Generally used for on-line transaction processing (OLTP) • Data Warehouses are a hybrid of databases which are used for On-line analytical processing (OLAP) Structure of a database External Schema Conceptual Schema Internal Schema Physical Schema External level • Level visible to user • Multiple views of the system – e.g. View an order - see limited product and customer information • Only the database Administrator may access the whole database at this level EXTERNAL SCHEMA • Each external view is defined by means of an external schema • Provides definitions of each external view. • Written in a Data Definition Language • individual to the user • accessed through a 3GL, a query language or a special purpose forms or menu-based language Conceptual level • CONCEPTUAL - represents the entire information content of the database • Consists of multiple types of conceptual record. This level preserves the data independence of the database. • CONCEPTUAL SCHEMA - defines each of the various types of conceptual record, in a conceptual Data Definition Language. Internal level • INTERNAL - a low-level representation of the entire database; it consists of multiple occurrences of multiple types of internal record. It is the stored record, inasmuch as it contains all but the device-specific information on the storage of the database. • PHYSICAL - the physical device and block addresses for each of the records. Mappings • Each level maps onto adjoining levels • conceptual / internal mapping specifies how conceptual records and fields are represented at the internal level • Changes can be made in the internal level without affecting the conceptual level • external / conceptual mapping defines the correspondence between an external view and the conceptual view DBMS - Database Management System • software handling access to the database • allows both the database administrator and all users the access to the database to which they are entitled How requests are processed • User issues request (e.g. through SQL) • DBMS intercepts and analyses request • DBMS inspects user's external schema, external to conceptual mapping, conceptual schema, conceptual to internal mapping and the storage structure definition. • DBMS executes operations on stored database. DATABASE ADMINISTRATOR (DBA) • Decide on the storage structure and access strategy • Liaise with the users • Define security and integrity checks • Define a backup and recovery strategy • Monitor and respond to performance Utilities used by the DBA • • • • • • Load routines Dump/Restore routines Reorganisation routines Statistics routines Analysis routines Data dictionary (containing METADATA, which gives data descriptions and mappings) Relational database • Data is independent from programs and from other data • Data is represented in TABLES rather than files. (one entity corresponds to 1 table) • Column headings are described as DOMAINS. (i.e. attributes) • Items of information as TUPLES or ROWS rather than records (i.e. occurrences of the entity)