Relational Algebra CS302 Week 6 Slides

Relational AlgebraCS302 Week 6 Slides

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

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

Is this a relation?

First name Last name Award Movie

Jeff Bridges Best Actor Crazy Heart

Sandra Bullock Best Actress The Blind Side

Kathryn Bigelow Director The Hurt Locker

Is this a relation?





Paul Ottosson Sound Editing The Hurt Locker

Paul Ottosson Sound Mixing The Hurt Locker

Is this a relation?






Paul Ottosson Sound Mixing Spider Man

Are these two relations the same?

Last name Award Movie First name

Bigelow Director The Hurt Locker

Kathryn

Bridges Best Actor Crazy Heart Jeff

Bullock Best Actress The Blind Side Sandra





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)

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;

Project exampleFirst name Last name Award Movie Birth year

Jeff Bridges Best Actor Crazy Heart 1949

Sandra Bullock Best Actress The Blind Side 1964

Kathryn Bigelow Director The Hurt Locker 1951

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


SQL: Select * from relation where condition;

Filter example

First name Last name Award Movie Birth year




What algorithm do you think Filter uses?

What algorithm do you think Filter uses? Trick question: depends on the DBMS and the

configuration!

Reduce changes the identifier, result:

narrower or the same width as the original relation possibly shorter than the original relation unsafe!


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

Reduce example





Paul Ottosson Sound Editing The Hurt Locker


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;

Group example

First name Last name Award Movie Birth year




Group example 2

First name Last name Award Birth year

Jeff Bridges Best Actor 1949

Sean Penn Best Actor 1960

Sandra Bullock Best Actress 1964

Kate Winslet Best Actress 1975

Getting started with GRAPE Start “X Server” ssh -YC [email protected] grape 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

mailto:[email protected]

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

Relational Algebra CS302 Week 6 Slides

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