What is a Database? Peter Wood

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1 Why study database management? 1. The database market is huge 2. There s a big demand for database skills 3. Managing data is a fundamental need for most applications ptw@dcs.bbk.ac.uk 4. The subject is interesting and challenging Web: s are used everywhere What is a? Essential for large amounts of data: banking, shopping,... (of course) scientific investigations, e.g. astronomical data, human genome but also used in smaller-scale applications: iphoto on Apple Macs uses sqlite3 to manage the photo library Squeezebox server uses mysql to manage the music library A database is a collection of persistent data A database models part of the real world A database is, in general, a shared resource

2 What is a DBMS? A DBMS is specialised software which is responsible for efficient storage and retrieval of large amounts of data in a database, allowing it to persist over long periods of time. (A DBMS is also referred to more simply as a database system.) Very simple database example Pubs: Sells: name location Horse and Hound Bloomsbury Hound and Hare Islington March Hare Bloomsbury Black Horse Islington White Horse Bloomsbury pub beer price Horse and Hound Bad Habit 1.50 Horse and Hound Rampant Ram 2.00 Hound and Hare Shining Wit 2.75 Hound and Hare Rampant Ram 2.50 March Hare Bad Habit 1.75 March Hare Rampant Ram 2.50 Black Horse Bad Habit 2.50 Black Horse Shining Wit 2.25 Black Horse Rampant Ram 2.50 White Horse Rampant Ram 2.75 Why Do We Need a DBMS? Three levels of Abstraction A DBMS is a software package that handles all the interaction of applications with the database. A DBMS hides details from the user/programmer using three levels of abstraction: 1. It saves programmer time by providing a declarative query language, e.g. SQL. 2. It saves programmer time by automatically checking constraints. 3. It saves maintenance time by ensuring data independence. 4. It provides concurrent access to the database for multiple, simultaneous users. 5. It provides automatic recovery from failure. 6. It provides security to ensure appropriate access to data. view 1 view 2 view n logical level physical level physical: how data is stored. logical: based on a data model. view: what programs or user see.

3 Data Independence Instances and Schemas Changes at one level of abstraction should not require changes at higher levels. physical data independence - the physical level may be changed without affecting the logical level. growth independence - the independence of the view level from the addition of new structures to the database. Deletions of structures at the logical level disrupt views that reference them. s change over time as information is inserted and deleted. The collection of data stored in a database at any moment in time is called an instance of the database. The overall design of the database is called the database schema. Schemas change infrequently. Physical schemas describe databases at the physical level. Logical schemas describe databases at the logical level. Schemas at the view level are sometimes called subschemas. Disadvantages of using a DBMS Types of DBMS users End users. Cost Lock-in Complexity Application programmers. administrator (DBA). Enterprise administrator - responsible for database design. Application administrator - responsible for view design.

4 DBMS examples Data Model DB2 Oracle A data model consists of three components: mysql 1. Structural part. Access 2. Integrity part. SQLserver 3. Manipulative part - declarative or procedural. various NoSQL systems, such as Cassandra, CouchDB, MongoDB The Relational Data Model Other Data Models entity-relationship model (next) 1. Structural part - relations. 2. Integrity part - keys (entity integrity) and foreign keys (referential integrity). 3. Manipulative part - Structured Query Language (SQL) or the relational algebra. hierarchical data model - from 1960s and 1970s network (CODASYL) data model - from 1960s and 1970s object-oriented data model - from 1980s and 1990s semi-structured data model (e.g. XML) - from 1990s document or key-value models (for NoSQL systems) - from 2000s

5 Summary Lecture Plan s are crucial to most organisations. A DBMS is a sophisticated piece of software that provides the interface between database users and the database itself. A DBMS has three levels of abstraction: physical, logical and view. Data independence means that changing data representation at a lower level does not affect a higher level. A data model has structural, integrity and manipulative parts. It is important to understand how 1. to access and update information in a database, e.g. using SQL 2. to design a database based on real-world constraints 3. a DBMS solves the problems associated with concurrent access and failure 1. to s 2. Data Modelling with the Entity-Relationship Model 3. The Basic Relational Model 3.1 Tables, Attributes and Domains 3.2 Primary and Foreign Keys Entity and Referential Integrity 4. Querying a Relational 4.1 Querying a Single Table 4.2 Querying Multiple Tables 4.3 Aggregating and Grouping Data 4.4 Updating Data 4.5 Null Values 4.6 Transaction Lecture Plan - Continued Recommended books 5. Designing a Relational 5.1 The three levels of a System and Data Independence 5.2 The Normalization Problem 5.3 Boyce-Codd Normal Form 5.4 Third Normal Form 6. s and the Web 1. [UW08] Covers most of the topics in this module, from a slightly theoretical or abstract perspective (> 500 pages) 2. [SKS11] Comprehensive introduction to (> 1300 pages). 3. [CB10] Another comprehensive and slightly less technical introduction to (> 1200 pages). There will be coursework! Apart from the above books, there are many more good database books at all levels.

6 T. Connolly and C. Begg. Systems: A Practical Approach to Design, Implementation, and. Addison-Wesley, fifth edition, A. B. Silberschatz, H. F. Korth, and S. Sudarshan. System Concepts. McGraw-Hill, sixth edition, J. D. Ullman and J. Widom. A First Course in Systems. Prentice Hall, third edition, 2008.

Introduction to Databases

Page 1 of 5 Introduction to Databases An introductory example What is a database? Why do we need Database Management Systems? The three levels of data abstraction What is a Database Management System?