Transcript PPT
CSCI-235 Micro-Computers in Science Databases • Database Concepts Data is any unorganized text, graphics, sounds, or videos A database is a collection of data that is multidimensional in the sense that internal links between its entries make the information accessible from a variety of perspectives • The Hierarchy of Data Field – a group of one or more characters that has a specific meaning • The smallest meaningful unit of data • Describes one characteristic of person, place, or thing Record – the set of fields containing data about a person, place, or thing File – a collection of related records • A file versus a database organization • Files System Approach Traditionally, information systems were developed with a file processing approach Resulted in a great deal of data redundancy, the repetition of the same data values • Increased the risk of inaccurate data • Increased the amount of storage space needed • Drawbacks of using file systems to store data • Data redundancy and inconsistency – Multiple file formats, duplication of information in different files • Difficulty in accessing data – Need to write a new program to carry out each new task • Atomicity of updates – Failures may leave database in an inconsistent state with partial updates carried out – Example: Transfer of funds from one account to another should either complete or not happen at all • Concurrent access by multiple users – Uncontrolled concurrent accesses can lead to inconsistencies – Example: Two people reading a balance and updating it at the same time • Example: ATM Bank Server Service a set of requests Do so without corrupting database Don’t hand out too much money • Database Management System (DBMS) DBMS contains information about a particular enterprise • Collection of interrelated data • Set of programs to access the data • An environment that is both convenient and efficient to use Database Applications • • • • • Banking: all transactions Airlines: reservations, schedules Universities: registration, grades Sales: customers, products, purchases Human resources: employee records, salaries Databases touch all aspects of our lives DBMS allows you to create a database and enter, modify, and retrieve data as needed Available at all levels • Sophisticated systems for a mainframe environment • Inexpensive, easy-to-use personal computer systems DBMS Features • • • • • Data maintenance Data retrieval Concurrency control Security Backup and recovery • Database Models Define the way a database organizes data Four main models • • • • Hierarchical Network Relational Object-oriented • Relational Database Each entity is stored in a separate table • Columns represent fields • Rows represent records Tables are linked by a relationship between primary and foreign keys • Primary Key A field that uniquely identifies a record • SalesID can be a primary key for the SalesPerson table • Once a SalesID appears in the table, no other salesperson can have that ID • Foreign Key A field in one table that is a primary key in another table • SalesID can be used in the Customer table to identify the salesperson who serves that customer • The same SalesID can appear in many customer records (a salesperson can serve many customers) • Relational Database Consists of multiple relations Information about an enterprise is broken up into parts, with each relation storing one part of the information account : stores information about accounts depositor : stores information about which customer owns which account customer : stores information about customers Storing all information as a single relation results in repetition of information (e.g., two customers own an account) • customer relation • account relation • depositor relation • Integrity Constraints Define acceptable values for a field • For example, the value of a month cannot be greater than 12 Primary keys cannot be duplicated Foreign keys cannot be used unless they exist as a primary key • A SalesID that is used in the customer table must exist as a primary key in the salesperson table