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From ER Diagrams to the Relational Model

From ER Diagrams to the Relational Model. Rose-Hulman Institute of Technology Curt Clifton. Review – Entity Sets and Attributes. Entity set: collection of “things” in the DB Attribute: property of an entity. calories. name. Soda. Kinds of Attributes. Simple – single atomic value

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From ER Diagrams to the Relational Model

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  1. From ER Diagrams to the Relational Model Rose-Hulman Institute of Technology Curt Clifton

  2. Review – Entity Sets and Attributes • Entity set: collection of “things” in the DB • Attribute: property of an entity calories name Soda

  3. Kinds of Attributes • Simple – single atomic value • Soda name, calories • Composite – several sub-attributes • PersonName(First,Middle,Last) • Multi-valued – set of values for one attribute • Car color, Degrees earned • (Somewhat rare, makes some searches harder)

  4. Review – Keys • Let us tell entities apart • The key for an entity set is a subset of the attributes for that entity set, such that no two entities agree on all the attributes calories name Soda

  5. Review – Relationships • Associate 2 or more entity sets • Constraints • Maximums shown with numbers • Read like: a subject-verb-number-object • Participation shown with double line • Read like: a subject-has to-verb… Most Likes N Person Soda 1

  6. ER Design Techniques • Avoid redundancy and don’t duplicate data • Don’t use entity set when attribute will do • Limit use of weak entity sets

  7. Redundancy • Wastes space • Leads to inconsistency • For example: manf addr name name Madeby N 1 Soda Manf

  8. Failed Attempt At Fix • Delete Manf entity set • Add address to Soda manf Manf addr name Soda

  9. Successful Fix • Eliminate manf attribute from Soda • Use relationship to find manufacturer info. addr name name Madeby N 1 Soda Manf

  10. Don’t Use Unnecessary Entity Sets • Entity set should… • Have at least one non-key attribute OR • Be the “many” in a many-one or many-many relationship addr name name Madeby N 1 Soda Manf

  11. Bad Entity Set • Suppose we didn’t have manufacturer address name name Madeby N 1 Soda Manf

  12. Avoid Weak Entity Sets • Don’t try to be too clever • Can usually just add a unique ID • Government has done this for their databases: • Social Security Numbers • Vehicle Identification Numbers • But… • Don’t trust uniqueness of IDs assigned by others

  13. Why Use Weak Entity Sets At All? • Federated Databases, for example… • All students in Indiana receiving state aid • All players on FIFA soccer teams • One query sent to multiple DB • Still want a Conceptual DB Schema • But no global authority to assign unique IDs

  14. The Relational Model • Originated as theoretical idea • “A Relational Model of Data for Large Shared Data Banks”, E. F. Codd, Comm. of the ACM, 13(6), June 1970 • http://www.acm.org/classics/nov95/s1p3.html • Revolutionized databases • Led to 1981 ACM Turing Award • The “Nobel Prize of computing”

  15. Relations • (Semi-) Formally • Tuple: an ordered list • n-tuple: an ordered list of length n • Relation: a set of n-tuples • Informally: • Relation: a table with unique rows • Rows = tuples; Columns = attributes; • Values in column = domain • Database: a collection of relations

  16. Some Other Terms • Relation schema • Describes a relation • RelationName (AttrName1, AttrName2,…) • Or RelationName (AttrName1:type, …) • Database schema • Set of all the relation schema for the DB’s relations

  17. Why is the Relational Model Dominant? • Very simple – just one data structure • Matches a “list the items” mentality • Easy to manipulate tables with UI • Forms basic foundation for SQL • Relational model based on sets • SQL based on bags (a.k.a., multi-sets)

  18. From ER Diagrams to Relations • Entity sets become relations • Columns are attributes of entity set • Relationships also become relations • Columns are keys of participating entity sets

  19. Example: Basic Entity Sets calories name addr Soda name Customer

  20. Example: Basic Relationship addr name calories name Likes Customer Soda MostLikes Namedbuddy 1 BuddyWith Naming buddy since

  21. Simplifying! • Can avoid relations formany-one relationships • Just add key of the one to the relation of the many Customer name addr MostLikes 1 calories name Soda

  22. Over Simplifying! • What happens if we try to eliminate relation for a many-many relationship? • Consider treating Likes as we did Most Likes • Redundancy • Data loss

  23. Need enough columns to make rows unique! So… All attributes of weak entity set + Key from supporting relationship Weak Entity Sets name number name city Plays on Player Team 1

  24. Entity Sets with Subclasses • Use nulls, or • Use multiple relations • “ER Style” • How should we choose which to use? name calories Soda isa sweetener DietSoda

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