1 / 11

# Database Systems {week 02} - PowerPoint PPT Presentation

Rensselaer Polytechnic Institute CSCI-4380 – Database Systems David Goldschmidt, Ph.D. Database Systems {week 02}. Selection (review). Find and select all tuples from relation R that satisfy some set of conditions Forms the basis of querying a database

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about ' Database Systems {week 02}' - charles-salas

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.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

CSCI-4380 – Database Systems

David Goldschmidt, Ph.D.

### Database Systems{week 02}

• Find and select all tuples from relation Rthat satisfy some set of conditions

• Forms the basis of querying a database

• The selectionC (R) is based on Boolean condition C over attributes of relation R

• Example conditions include:

• A = e, A > e, A >= e, A < e, A <= e, A <> e

• A1 = A2, A1 <> A2

• Any combination of conditions using AND, OR, NOT

A, A1, and A2 are attributes

e is a constant or expression

• Selection selects a subset of tuples in relation R (with the schema unchanged)

• C(R) = { t | t is a tuple in R and t satisfies the condition C on relation R }

• Selection conditions can only refer toattributes in the given relation R

• For conditions spanning multiple relations, we first must combine those relations (i.e. join)

• The Cartesian product combinestwo relations to form a new relation

• The new relation has all of the attributesof the original two relations

• It’s often a good idea to rename attributesin the original relations such thatthere’s no ambiguity

• Given two relations R and S withschemas R(A1,A2,...,An) and S(B1,B2,...,Bm)

• The Cartesian product RxS producesrelation T with schema T(A1,A2,...,An,B1,B2,...,Bm)

• We can disambiguate attributesby using R.Ax and S.By

• i.e. attributes Ax and By have the same name

• The Cartesian product of relations R and S:

• RxS = { (r,s) | r is a tuple in R and s is a tuple in S }

• Note that (r,s) has all values in r and s

• The Cartesian product is like multiplicationin that it produces a tuple for every pair oftuples from R and S

• e.g. if R has 10 tuples and S has 5 tuples, then RxS will have 50 tuples

So why do we want to do this?

• Cartesian product RxS is often followed by a selection condition that specifies how tuples in R should be matched to tuples in S

• Translation: Join the Undergraduate and Advising relations by selecting tuples in which the name attribute equals the student attribute

• When joining two relations, we often want to join on common (same-named) attributes

• This is a natural join on relations R and Sand is denoted R⋈S

• The selection condition selects tuples that have the same values for same-named attributes

• Note that the schema does not repeat thesame-named attributes

• Joins based on specific conditions arecalled theta joins and are denoted R ⋈C S

• To perform a theta join, first take theproduct of relations R and S

• Next, select tuples that satisfy condition C

• The resulting schema is the union of theschemas of R and S with R.Ax and S.By prefixes,if necessary

In a theta join, nodeduplicationof attributes is performed!

• The rename operator changes thename of the attributes of relation Rwith schema R(A1,A2,...,An)

• S(B1,B2,...,Bn)(R) = relation S(B1,B2,...,Bn) in which R.A1 is renamed S.B1, R.A2 is renamed S.B2, and so on

• To only change some attribute names,simply specify Bi = Ai

• Find all faculty who advise a student

• Find all faculty who do not adviseany students

• Find faculty who advise at least two students

• Find faculty who advise a student that is not in their department (e.g. dual majors)