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SQL Queries

SQL Queries. Database. Database is SQL1.mdb import using MySQL Migration Toolkit http://www.mysql.com/products/tools/. SQL1 Database. Based on tables used in “The Essence of SQL” , by David Rozenshtein Five tables STUDENT PROFESSOR COURSE TAKE TEACH Three types of SQL queries

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SQL Queries

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  1. SQL Queries

  2. Database • Database is SQL1.mdb • import using MySQL Migration Toolkit • http://www.mysql.com/products/tools/

  3. SQL1 Database • Based on tables used in • “The Essence of SQL”, by David Rozenshtein • Five tables • STUDENT • PROFESSOR • COURSE • TAKE • TEACH • Three types of SQL queries • Eighteen standard questions • “Essence” discusses methodology, however does not provide any data • SQL1 database contains data according to the book’s table definitions • Data is ‘salted’ (known results for the queries)

  4. SQL query facilities • Can be lumped into six categories • Type I, II, or III categories • Type I: single level queries • Type II: nested, no correlations • Type III: nested, with correlations • Aggregation • sums, averages, grouping • Enhancements • sorting • Extensions • WHILE loop

  5. Standard Questions • E1. Who takes CS112? • E2. What are the student names and numbers of students taking CS112? • E3. Who takes CS112 or CS114? • E4. Who take both CS112 and CS114? • E5. Who does not take CS112? • E6. Who takes a course which is not CS112? • E7. Who takes at least two courses? • E8. Who takes at most two courses? • E9. Who takes exactly two courses? • E10. Who takes only CS112? • E11. Who takes either CS112 or CS114? (not both) • E12. Who are the youngest students? • E13. Who takes every course?

  6. E1 and E2 • Type 1 queries • basic form has three clauses • SELECT <fieldlist> • FROM <tablelist> • WHERE <Boolean condition> • E1 (Who takes CS112?) SELECT sno FROM Take WHERE (cno=“CS112”); • E2 (Who takes CS112? – show name and number) SELECT Student.sno, Sname FROM Student,Take WHERE (Student.sno=Take.sno) AND (cno=“CS112”);

  7. E1 and E2 answers

  8. E3: “or” (inclusive) • Who takes CS112 or CS114?

  9. E3 • “distinct” keyword requires sorting • performance hit • rewrite the WHERE clause to use list membership operator “IN” • IN returns true is the value on the left is equal to one of the value list members on the right • WHERE (cno IN(“CS112”,”CS114”)) • The “IN” operator is important for Type II queries!

  10. E4: both… • Who takes both CS112 and CS114? SELECT sno FROM take WHERE (cno=“CS112”) and (cno=“CS114”); • Why is this wrong? • it will compile and return an answer… so.. why? • because it tries to find any entry for which the course number is CS112 and CS114… • it can only be one at a time… • translation from English to SQL needs to be considered • rephrase the question? • reverse engineer the problem?

  11. E4 • The information resides in the ‘take’ table • find students who take CS112 (who takes one) • find students who take CS114 (who take the other) • determine if it’s the same student (where are they equal) • Ideally use another ‘copy’ of the take table SELECT take.sno FROM take, take WHERE (take.sno=take.sno) AND (take.cno=“CS112”) and (take.cno=“CS114”); • however…. how does the compiler know which take table to use when??? • Develop an ‘alias’ for one of the take tables, and then use the alias name!

  12. E4 • Alias • created in the FROM clause and used only for the duration of the query SELECT X.sno FROM take X, take WHERE (X.sno=take.sno) AND (X.cno=“CS112”) and (take.cno=“CS114”);

  13. E5 and E6 (negation) • E5: Who does not take CS112? • E6: Who takes a course which is not CS112?

  14. E6 • Who takes a course which is not CS112? • (student will be selected even if takes CS112 in addition to another course) SELECT sno FROM take WHERE (cno!=“CS112”);

  15. E5 • Who does not take CS112? • A Type II query • done in two passes, not one (subquery) • 1st pass • who takes CS112? • 2nd pass • who is not in the results of the first pass?

  16. E7 • Who takes at least two classes? • Last of the Type I queries • Use two “copies” of the TAKE table • See who exists in both copies, but have DIFFERENT classes • The student number the same, but differ class numbers SELECT x.sno FROM take x, take y Where (x.sno=y.sno) AND (x.cno != y.cno);

  17. Type I queries • Questions 1,2,3,4,6,7 can be answered with Type I queries • Question 5 is a Type II query and cannot be answered with a Type I query

  18. Type II queries • E8 through E11 • E8: Who takes at most two courses? SELECT sno FROM student WHERE NOT (sno IN (Select x.sno From Take x, Take y, Take Where (x.sno=y.xno) And (y.sno=take.sno) And (x.cno!=y.cno) And (y.cno !=take.cno) And (x.cno!=take.cno)));

  19. E9 • Who takes exactly two courses • Who takes at least two courses and does not take at least three courses? Note: “x” alias used twice in main and subquery, but “scope” keeps them separate The “>” used in the where clause is used to eliminate duplicates

  20. E10 • Who takes only CS112? • Who takes CS112 and is not in the group of who does not take CS112? SELECT sno FROM take WHERE (cno=“CS112”) AND NOT (sno IN (SELECT sno FROM take WHERE (cno != “CS112”))); Alternate: simplified query

  21. E11 • Who take either CS112 or CS114? • But not both… • Find who takes both and from that • Find who is NOT in that group, but takes CS112 or CS114 SELECT sno FROM take WHERE ((cno=“CS112”) or (cno=“CS114”)) AND NOT (sno IN (SELECT x.sno FROM take x, take WHERE (x.sno=take.sno) AND (x.cno=“CS112) AND (take.cno=“CS114))); NOTE: extra parens around the “OR” In the WHERE clause. This means that the OR is executed first;. Normal Precedence: NOT, AND OR

  22. E12: Extremes with negation • E12: Who are the youngest students? • The min( ) function can be used or a TYPE II can do it as well SELECT sno FROM student WHERE NOT (age IN (SELECT x.age FROM student x, student WHERE (x.age > student.age))); • The subquery returns all ages if there is age smaller than it • It excludes the smallest age • Then the main query retrieves the students who are not in that group

  23. Features

  24. E13 • Who takes every course? • Note the use of the CONCAT( ) function • Allows us to make up a new “field” for the duration of the query • Saves comparing multiple fields

  25. E13 • Local reference in each level of the query • This makes it a TYPE II • Innermost sno,cno comes from TAKE • Middle sno comes from cross-product of STUDENT,COURSE • Outermost sno comes from STUDENT

  26. E14 • For each department that has more than 2 professors older than 40, what is the average salary of these professors? Selectdept,avg(salary) From professor Where (age>40) Groupby dept Having (count(*)>2);

  27. Aggregate Functions • Count(*) from professor • Counts the rows • Count(Distinct(dept)) from professor • Counts how many unique values exist • Count would return • When using aggregate functions (min, max, avg, sum, count, etc) you cannot have a non-aggregated column in the SELECT or HAVING clause unless it’s already aggregated in the GROUP BY clause

  28. E15: Aggregate and Scalar functions • What is the GPA of each student? Select sno, GPA=round(SUM(Grade*credits)/Sum(credits),2) FROM take,course WHERE (take.cno=course.cno) GROUP BY sno;

  29. Using aggregates • TYPE I question involving “at least, at most, exactly” can be performed using aggregate SELECT sno FROM take GROUP BY sno HAVING (count(*)>=3);

  30. E16: Global aggregations • What is the overall average salary of all professors older than 40? SELECT AVG(salary) FROM professor WHERE (age>40); • GROUP BY clause not needed • Implicit ‘single group’ of all professors over 40

  31. E17 • Which professor’s salary is greater than the overall average salary? SELECT x.fname,x.lname FROM professor x, professor y GROUP BY x.fname,x.lname,x.salary HAVING (x.salary > avg(y.salary));

  32. E18 • Which professor’s salary is greater than his/her department’s average salary? SELECT x.fname,x.lname FROM professor x,professor y WHERE (x.dept=y.dept) GROUP BY x.fname,x.lname,x.salary HAVING (x.salary > avg(y.salary));

  33. TYPE III Queries • Sample Type III SELECT sno,sname FROM student WHERE (“CS112” IN (SELECT cno FROM take WHERE (sno=student.sno))); • What makes this a Type III ? • The reference in the inner where [take.]sno = student.sno • “reaches” outside the scope of the inner query • It is a “non-local reference” or “correlation” • It’s E2… What are the student numbers and names of the students taking CS112?

  34. TYPE III • Who teaches CS112?

  35. TYPE III • Who takes CS112? • Using the EXISTS operator • Returns TRUE if at least one row returned by subquery SELECT sno,sname FROM student WHERE EXISTS (SELECT * FROM take WHERE (sno=student.sno) AND (cno=“CS112”)); EXISTS requires usage of the “*” in the select subquery

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