1. Relational Model CMSC 461 Michael Wilson

2. What is a relation?  A relation is a set of tuples  (d 1, d 2, d 3, …, d n )  Where each element d i is a member of D i, which is the data domain for d i  The order of the tuples in the set is irrelevant  A data domain describes all unique values that a particular data element may contain  Corresponds to data types  Each element is called an attribute value

3. A picture jacked shamelessly from Wikipedia

4. An example UMBC IDAgeFNameLNameGPA SM1111120Mario 4.0 SM1111219LuigiMario3.9 MT1111121SamusAran4.0 FF1111321SnowVilliers0.2 SW1123418SnowWhite3.5

5. Relation schemas  Relation schemas are basically groups of named attributes along with any constraints on the data housed by the attribute  For the previous example, the relation schema would be:  UMBC ID, Age, FName, LName, GPA  String, Integer, String, String, Double

6. Translating to database terminology  A relation looks a lot like a table  An attribute looks a lot like a column  A tuple looks a lot like a row  Finally, a database contains many tables  A database can be thought of as a set of relations

7. Differences between relations and tables  Very minor stuff  A table can have duplicate rows, whereas a relation cannot have duplicate tuples  Tables have some metadata associated with them for programmatic reasons

8. Identification of tuples  To identify a tuple, typically we’ll take a subset of the values in the tuple in order and refer to it that way  Any subset of values that can uniquely identify the tuple is called a superkey  Uniquely identifying a tuple requires that the superkey be unique  As long as the subset itself is unique, it works  Even if one or more values are shared between superkeys

9. Superkeys! UMBC IDAgeFNameLNameGPA SM1111120Mario 4.0 SM1111219LuigiMario3.9 MT1111121SamusAran4.0 FF1111321SnowVilliers0.2 SW1123418SnowWhite3.5

10. Superkeys! UMBC IDAgeFNameLNameGPA SM1111120Mario 4.0 SM1111219LuigiMario3.9 MT1111121SamusAran4.0 FF1111321SnowVilliers0.2 SW1123418SnowWhite3.5

11. More superkeys! Record IDPatientNameDate of VisitDiagnosis 14233Mario 04/28/1991Leg injury 174342Snow Villiers12/05/2012Brain damage 189232Lara Croft12/05/2012Powder burns

12. More superkeys! Record IDPatientNameDate of VisitDiagnosis 14233Mario 04/28/1991Leg injury 174342Snow Villiers12/05/2012Brain damage 189232Lara Croft12/05/2012Powder burns

13. More superkeys! Record IDPatientNameDate of VisitDiagnosis 14233Mario 04/28/1991Leg injury 174342Snow Villiers12/05/2012Brain damage 189232Lara Croft12/05/2012Powder burns

14. More superkeys! Record IDPatientNameDate of VisitDiagnosis 14233Mario 04/28/1991Leg injury 174342Snow Villiers12/05/2012Brain damage 189232Lara Croft12/05/2012Powder burns

15. What use is a superkey?  In the previous examples, we’ve got multiple possible superkeys  What use are they?  Allows us to introduce a secondary concept, really

16. Candidate keys  A candidate key is a superkey that contains the fewest number of values that can uniquely identify a tuple  A relation can have multiple candidate keys

17. Candidate keys? Record IDPatientNameDate of VisitDiagnosis 14233Mario 04/28/1991Leg injury 174342Snow Villiers12/05/2012Brain damage 189232Lara Croft12/05/2012Powder burns

18. Choosing a candidate key  A candidate key is a superkey that contains the fewest number of values that can uniquely identify a tuple  A relation can have multiple candidate keys  How do you know which one to choose?

19. Choosing a candidate key  This is actually somewhat of an art  The answer, really, is to choose the one that best suits your data  Looking at ER Diagrams can help you decide

20. Primary keys  When you’ve chosen a candidate key to use, you’ve chosen a primary key  The primary key is the way you reference items in your relation  There will be much more on this later

21. Some examples  phpBB - https://wiki.phpbb.com/Tableshttps://wiki.phpbb.com/Tables  phpBB uses 63 tables