IST 210. Databases and DBMSs. Todd S. Bacastow January 2005

Transcription

1 Databases and DBMSs Todd S. Bacastow January

2 Evolution

3 Ways of storing data Files (1960) (ancient times) Databases Hierarchical (1970) Network (1970) Relational (1980) Object (1990)

4 File terms Record data items related to a single logical entity (e.g. a student s information) or row in a table Field File a place for a data item in a record (first name field in a student record) or a column in a table a sequence of records of the same type (the table)

5 File structures A file: STUDENT field ID Last First Grade 3 Smith Jane A record 1 Wood Bob C 2 Kent Chuck B 4 Boone Dan B

6 File organization Serial new records appended Sequential records ordered in file, usually according to a numeric key

7 File structures Serial list of entries in which the order of entry into the list determines the order of the list ID Last First Grade 3 Smith Jane A 1 Wood Bob C 2 Kent Chuck B 4 Boone Dan B

8 File structures Search of a simple list entails going through each record until the query is satisfied (linear search), which is inefficient ID Last First Grade 3 Smith Jane A 1 Wood Bob C 2 Kent Chuck B 4 Boone Dan B

9 File structures Sequential list of entries ordered in some way (e.g. numerically or alphabetically) ID Last First Grade 1 Wood Bob C 2 Kent Chuck B 3 Smith Jane A 4 Boone Dan B

10 File structures Search of an ordered sequential list can use a search method ID Last First Grade 1 Wood Bob C 2 Kent Chuck B 3 Smith Jane A 4 Boone Dan B

11 File structures Indexes provide a reference to records based on an index field, which is ordered Last Pointer Boone * Kent * Smith * Wood * ID Last First Grade 1 Wood Bob C 2 Kent Chuck B 3 Smith Jane A 4 Boone Dan B

12 Problems with files Redundancy number of files grows with applications, and data is duplicated Inconsistency data is updated in one application s files, but not in another s Maintenance problems changes to data structures mean changes to many programs Difficulty combining data business needs may mean users want data from different applications

13 Other ways to organize Data model A data model is a particular way of conceptually organizing multiple data files in a database Other common models Hierarchical Network Relational Object

14 Network model

15 Network data model Relationships: one-to-one one-to-many many-to-one many-to-many Student Class Instructor Grade ID Department

16 Network data model Advantages flexible, fast, efficient Disadvantages Complex Restructuring can be difficult because of changing all the pointers

17 Hierarchical database model

18 Hierarchical data model Parent-child relationship: one-to-one one-to-many Student Class Instructor Grade ID Department

19 Hierarchical data model Advantages easy to search add new branches easily Disadvantages Must establish the types of search prior to development of the hierarchical structure

20 Summary Hierarchical and network data models have generally been replaced by the relational data model Relational databases dominate the database market Oracle Informix SQL Server DB2..

21 Relational database model Stores both Data about real world objects (entities) in tables Relationships between the tables

22 Relational database Fields (columns) in the table store attributes. Each attribute has a specific domain. Tuples (or records or rows) in the table store information. Each tuple is a unique instance of an object. Tables are composed of a set of tuples. A table is also called a relation.

23 Terms Table A collection of relevant data relating to one type of real world objects. Column A specific place for one type of data relating to one type of real world objects. Domain Set of all possible values for a specific column. Row Collection of data describing one real world object. Primary Key Columns, which are part of the row and uniquely identify any one row.

24 Records Each record represents a logical entity (e.g. a student) Each field represents an attribute of the logical entity Student ID Last First Grade Class 1 Wood Bob C IST357 2 Kent Chuck B IST115 3 Smith Jane A IST357 4 Boone Dan B IST357

25 Keys Each table has a primary key, one field (or a combination of fields) that has a unique value for each and every record in the table ID is the primary key in this table (two students may share either a last or first name) Student ID Last First Grade Class 1 Wood Bob C IST357 2 Kent Chuck B IST115 3 Smith Jane A IST357 4 Boone Dan B IST357

26 Relating tables Tables can be related (joined) together based on their keys The idea is to decompose into separate tables with no redundancy and to provide a capability to reassemble with no information loss Student ID Last First Grade Class 1 Wood Bob C IST357 2 Kent Chuck B IST115 3 Smith Jane A IST357 4 Boone Dan B IST357 Class Name #Stud Instructor IST Jones IST Brower IST Fountain

27 Relating tables Primary key Foreign key Primary key Student ID Last First Grade Class 1 Wood Bob C IST357 2 Kent Chuck B IST115 3 Smith Jane A IST357 4 Boone Dan B IST357 Class Name #Stud Instructor IST Jones IST Brower IST Mennis

28 Relating tables Student ID Last First Grade Class 1 Wood Bob C IST357 2 Kent Chuck B IST115 3 Smith Jane A IST357 4 Boone Dan B IST357 Class Name #Stud Instructor IST Brower IST Jones IST Jones Instructor Name Office Jones 332 Brower 517

29 DBMS Schema Ultimately data in databases is stored in files, but their structure is hidden External Schema Conceptual Schema Internal Schema The view on data used by application programs. The logical model of data that is separate from how it is used. The physical storage of data in files and indexes.

30 RDBMS Features Data Definition Language (DDL) Data Manipulation Language (DML) Integrity Constraints Transaction Management Concurrency Security Tuning of Storage

31 Data integrity and validation (Constraints)

32 Relationships Link between entities. A relationship may define constraints. E.G, a person can only have one SSN.

33 Advantages of RDBMS Eliminate unnecessary duplication of data Enforce data integrity through constraints Changes to conceptual schema need not affect external schema Changes to internal schema need not affect the conceptual schema Many tools are available to manage the database

34 Disadvantages of RDBMS To store objects (e.g., drawings) in a relational database, the objects have to be flattened into tables e.g., a digital representation of a parcel must be separated from the behaviour of other parcels Complex objects have to be taken apart and the parts stored in different tables When retrieved from the database, the object has to be reassembled from the parts in different tables

35 Other Types of DBMS Object DBMS store objects as objects designed to handle complex nested objects for graphical and multimedia applications Object-relational DBMS hybrid databases that can store data in tables but can also store objects in tables

36 Object DBMS ODBMS have the advantage that objects can be stored directly Object databases are closely linked to programming languages with ways of navigating through the database

37 Summary Common word Textbook word Alternate word Table Relation File or Data set (old) Object word Object class Column Attribute Field Object field Domain Domain Range of possible values Datatype, subtype Row Tuple Record Object instance Primary key Primary key Key of the record Object identifier