Solution: Homework 2: Relational Algebra Due at 5pm on Monday, February 5, 2018

Transcription

1 CS 500, Fundamentals of Databases, Winter 2018 Solution: Homework 2: Relational Algebra Due at 5pm on Monday, February 5, 2018 Description This assignment covers relational algebra. When writing relational algebra statements, keep in mind that your answer should be correct for any valid instance of the given database schema. In other words you are writing programs that should compute correctly on any valid input, not only on the particular instance you are given as an example. Relational algebra queries can be written by hand, in LaTex or using any other editor / typesetting system of your choice. We prefer to receive this part of your assignment as a PDF file. Name your file hw2.pdf, or use an appropriate file extension. Details and Grading This assignment is made up of 2 problems, collectively worth 90 points, or 5% of the over-all course grade. If this assignment is submitted late, you will receive no credit. This assignment is to be completed individually. Please consult the course syllabus for a description of our academic honesty policy. Submission instructions Submit your assignment to gitlab on gitlab.cci.drexel.edu. I assume that you already followed the steps to create your git repository. I will refer to the root directory of your git repository as $GIT_HOME. Create a directory called cs500-hw2 case-sensitive, use exactly this name) under $GIT_HOME: mkdir $GIT_HOME/cs500-hw2 cd $GIT_HOME/cs500-hw2 Place the file hw2.pdf into this directory. You can now commit your assignment as follows: git add * git commit -m homework 2 git push You may submit multiple times before the deadline, only your last submission committed before the deadline will be graded. 1

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3 Part 1 30 points): Relational Algebra Consider relation instances on page 2, with the given schemas. In each question below, write a relational algebra expression that computes the required answer. For readability, abbreviate relation names as follows: C for Candidates, J for Jobs, JS for Jobs_Skills, etc. Don t worry about renaming attribute names in the result, e.g., there is not need to explicitly rename the attribute C.salary as candidate_salary. a) For each position, list all candidates whose salary requirements meet the salary budget. Result should have the schema job, grade, company, candidate, candidate_salary). π P. job, P.grade, P.company, C.candidate, C.salary P P.salary C.salary C) b) Are there job skills with no qualified candidates in our database? A candidate who has the skill but not the required number of years of experience is not qualified for the skill. The query should list all such skills, with the schema skill, years). π skill,years JS) π JS.skill, JS.years JS JS.skill=CS.skill JS.year CS.years CS)) c) List pairs of candidates who both have at least 3 years of DB experience. Return each pair of candidates exactly once, i.e., do not return both 'Ann', 'Cathy') and 'Cathy', 'Ann'). Do not return a pair that contains the same candidate twice, e.g., do not return 'Ann', 'Ann'). Result should have the schema candidate1, candidate2). π CS1.candidate, CS2.candidate ρ CS1 σ skill='db' years 3 CS)) CS1.candidate< CS2.candidate ρ CS2 σ skill='db' years 3 CS))) 3

4 Tennis_Players name, country, ATP_rank, age, points) name country ATP_rank age points Djokovic Serbia Murray UK Federer Switzerland Nadal Spain Wawrinka Switzerland Nishikori Japan Raonic Serbia Years_Ranked_First name, year) name year Djokovic 2015 Djokovic 2014 Nadal 2013 Djokovic 2012 Djokovic 2011 Nadal 2010 Federer 2009 Nadal 2008 Countries name, GDP, population) name GDP B) population M) USA 18, China 11,383 1,383 Japan 4, Germany 3, UK 2, Spain 1, Switzerland Serbia 37 9 Federer 2007 Federer 2006 Federer 2005 Federer

5 Part 2 60 points): Relational Algebra Consider relation instances on the previous page, with the given schemas. In each question below, write a relational algebra expression that computes the required answer. a) List names of home countries of tennis players who were ranked first between 2013 and 2010 inclusive). π country σ 2010 year 2013 YRF)) name b) List names and GDPs of countries from which there are no tennis player in our database. π name, GDP C) π name, GDP C C.name=TP.country c) List pairs of tennis players such that i) the ATP rank of the first is lower better) than that of the second, and ii) the GDP of his home country is lower than that of the second. σ P1.ATP_rank<P2.ATP_rank) P1.GDP<P2.GDP) ρ P1 TP TP.country=C.name C) ρ P2 TP TP.country=C.name C)) d) List name, age, ATP rank and country s GDP of tennis players from Spain or Serbia. π TP.name, TP.age, TP.ATP _ rank, C.GDP σ name='spain' name='serbia' C)) C.name=TP.country e) List name, ATP rank and country of tennis players who were ranked first in 2010 or later but not before π name, ATP _ rank, country π name σ 2010 year YRF)) π name σ 2010>year YRF))) name f) List names and populations of countries of tennis players who are currently ranked 5 or lower better), are currently 30 years old or older, and were ranked first in some year since 2004 including 2004). π C.name, C.population σ ATP_rank 5 age 30 name σ year 2004 YRF))) TP.name=C.name C) 5