1. Relational Algebra CS302 Week 6 Slides

2. SQL is the language used for talking to a relational database management system Allows changing the schema (“data definition”), changing the data (“data modification”), and querying But the querying part of SQL isn't very good  Confusing syntax that is hard to read  Major differences from one DBMS to the next We teach you relational algebra (RA) for querying  We use a graphical representation of RA  It's easy to convert RA into SQL by hand  We'll use GRAPE to automatically convert RA into SQL too

3. A relational database is a set of relations A relation is a set of rows and a set of columns A relation has an identifier which is a set of columns We consistently underline columns in the identifier Rules: A row has a value for each column in the identifier  (A row may or may not have values for columns that are not in the identifier) No two rows have the same values in all the identifying columns

4. Is this a relation? First nameLast nameAwardMovie JeffBridgesBest ActorCrazy Heart SandraBullockBest ActressThe Blind Side KathrynBigelowDirectorThe Hurt Locker

5. Is this a relation? First nameLast nameAwardMovie JeffBridgesBest ActorCrazy Heart SandraBullockBest ActressThe Blind Side KathrynBigelowDirectorThe Hurt Locker

6. Is this a relation? First nameLast nameAwardMovie JeffBridgesBest ActorCrazy Heart SandraBullockBest ActressThe Blind Side KathrynBigelowDirectorThe Hurt Locker PaulOttossonSound EditingThe Hurt Locker PaulOttossonSound MixingThe Hurt Locker

7. Is this a relation? First nameLast nameAwardMovie JeffBridgesBest ActorCrazy Heart SandraBullockBest ActressThe Blind Side KathrynBigelowDirectorThe Hurt Locker PaulOttossonSound MixingThe Hurt Locker PaulOttossonSound MixingSpider Man

8. Are these two relations the same? Last nameAwardMovieFirst name BigelowDirectorThe Hurt Locker Kathryn BridgesBest ActorCrazy HeartJeff BullockBest ActressThe Blind SideSandra First nameLast nameAwardMovie JeffBridgesBest ActorCrazy Heart SandraBullockBest ActressThe Blind Side KathrynBigelowDirectorThe Hurt Locker

9. RA Operators We'll look at unary operators first These take a relation and return a relation project filter reduce group (Unary Operators, p. 72, examples 1-8 work on one relation)

10. Project Project removes and adds non- identifying columns Adds computed columns – arithmetic, string manipulation, etc. result: as wide as number of columns named,  which must include the identifying columns exactly as tall as original relation identifier is same see helper table, p. 74, Unary Ops : note where above info is SQL: Select columns from relation;

11. Project example First nameLast nameAwardMovieBirth year JeffBridgesBest ActorCrazy Heart1949 SandraBullockBest ActressThe Blind Side1964 KathrynBigelowDirectorThe Hurt Locker1951

12. Filter Filter removes rows based on a condition result: as wide as the original relation shorter or exactly as tall as the original relation identifier is same see helper table, p. 74, Unary Ops : note where above info is SQL: Select * from relation where condition;

13. Filter example First nameLast nameAwardMovieBirth year JeffBridgesBest ActorCrazy Heart1949 SandraBullockBest ActressThe Blind Side1964 KathrynBigelowDirectorThe Hurt Locker1951

14. What algorithm do you think Filter uses?

15. Trick question: depends on the DBMS and the configuration!

16. Reduce changes the identifier, result: narrower or the same width as the original relation possibly shorter than the original relation unsafe! see helper table, p. 74, Unary Ops : note where above info is SQL: Select distinct new_identifier from old_identifier Note: Our book also allows reduce with carried non-identifying columns, but this is trickier to do in SQL Reduce

17. Reduce example First nameLast nameAwardMovie JeffBridgesBest ActorCrazy Heart SandraBullockBest ActressThe Blind Side KathrynBigelowDirectorThe Hurt Locker PaulOttossonSound EditingThe Hurt Locker PaulOttossonSound MixingThe Hurt Locker

18. Group Group changes the identifier, combining duplicate rows using an aggregate function result: narrower, the same width, or wider than the original relation possibly shorter than the original relation unsafe! SQL: select new_identifier, aggregate_function from relation group by new_identifier Select distinct columns from relation;

19. Group example First nameLast nameAwardMovieBirth year JeffBridgesBest ActorCrazy Heart1949 SandraBullockBest ActressThe Blind Side1964 KathrynBigelowDirectorThe Hurt Locker1951

20. Group example 2 First nameLast nameAwardBirth year JeffBridgesBest Actor1949 SeanPennBest Actor1960 SandraBullockBest Actress1964 KateWinsletBest Actress1975

21. Getting started with GRAPE Start “X Server” ssh -YC grape@grape.dfeldman.org grapegrape@grape.dfeldman.org Log in:  Usernames s10carlis1 – s10carlis15  Passwords are the same  Pick an account and change the password please Start the SQL Shell and load in some data! Web site about GRAPE: csrg.cs.umn.edu/grape Carlis's Tiny DB is on that site as well

22. Your Homework Go to sqlzoo.net  If you have not used relational databases before, do Tutorials 1-4  If you have used relational databases before: start where you're comfortable and try to finish all the tutorials Install GRAPE on your personal machine if possible