lec10.365.worksheet

lec10.365.worksheet - Fall 2007 CPE/CSC 365 Introduction to...

Info icon This preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
. . Fall 2007 CPE/CSC 365: Introduction to Database Systems Alexander Dekhtyar . . SQL: Structured Query Language Grouping Queries: Example How Grouping works Consider two relations R(A,B,C) and S(B,D) (all attributes are integer). Con- sider the following instances of tables R and S : R A B C 1 2 2 1 4 4 2 2 5 3 4 8 2 4 1 3 4 12 S B D 2 1 4 7 Consider the following SQL query SELECT R.A, SUM(R.B), AVG(R.C), COUNT(*), MIN(S.D) FROM R, S WHERE R.B = S.B and R.C < 10 GROUP BY R.A HAVING COUNT(*) > 1; This query is evaluated as follows. Step 1. FROM Clause: Cartesian Product. First, the FROM clause is evalu- ated. It is convenient to view thist step as creation of a cartesian product of all tables referenced in the FROM clause. Here, this leads to computation of R × S : 1
Image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
R × S R.A R.B R.C S.B S.D 1 2 2 2 1 1 4 4 2 1 2 2 5 2 1 3 4 8 2 1 2 4 1 2 1 3 4 12 2 1 1 2 2 4 7 1 4 4 4 7 2 2 5 4 7 3 4 8 4 7 2 4 1 4 7 3 4 12 4 7 Step 2: WHERE Clause: Join and selection. On this step, each tuple in the cartesian product constructed on the previous step is evaluated against the conditions specified in the WHERE clause.
Image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern