1 / 22

Databases and DBMSs

Databases and DBMSs. Todd S. Bacastow January 2005. 1. Evolution. Ways of storing data. Files (1960) (ancient times) Databases Hierarchical (1970) Network (1970) Relational (1980) Object (1990). File terms. Record

kessie-floyd
Télécharger la présentation

Databases and DBMSs

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Databases and DBMSs Todd S. Bacastow January 2005 1

  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 • a place for a data item in a record (first name field in a student record) or a column in a table • File • 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 1 Wood Bob C 2 Kent Chuck B 4 Boone Dan B record

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

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

  8. 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

  9. 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

  10. Network model

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

  12. Network data model Relationships: • one-to-one • one-to-many • many-to-one • many-to-many Class Student Instructor Department Grade ID

  13. Hierarchical database model

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

  15. 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

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

  17. 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.

  18. 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

  19. 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 Student ID Last First Grade Class ID is the primary key in this table (two students may share either a last or first name) 1 Wood Bob C IST357 2 Kent Chuck B IST115 3 Smith Jane A IST357 4 Boone Dan B IST357

  20. 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 Class ID Last First Grade Class Name #Stud Instructor 1 Wood Bob C IST357 2 Kent Chuck B IST115 3 Smith Jane A IST357 4 Boone Dan B IST357 IST357 48 Jones IST115 120 Brower IST20 120 Fountain

  21. Relating tables Primary key Foreign key Primary key Student Class ID Last First Grade Class Name #Stud Instructor 1 Wood Bob C IST357 2 Kent Chuck B IST115 3 Smith Jane A IST357 4 Boone Dan B IST357 IST357 48 Jones IST115 120 Brower IST20 120 Mennis

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

More Related