 Database is SQL1.mdb ◦ import using MySQL Migration Toolkit 

 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)

 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

 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?

 Type 1 queries ◦ basic form has three clauses  SELECT  FROM  WHERE  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”);

 Who takes CS112 or CS114?

 “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!

 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?

 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!

 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”);

 E5: Who does not take CS112?  E6: Who takes a course which is not CS112? Included in E5?Included in E6? Student takes both 112 and 114 NoYes Student takes nothingYesNo Only takes 112No Only takes 114Yes

 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”);

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

 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);

 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

 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)));

 Who takes exactly two courses ◦ Who takes at least two courses and does not take at least three courses? Note: 1)“x” alias used twice in main and subquery, but “scope” keeps them separate 2)The “>” used in the where clause is used to eliminate duplicates

 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

 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

 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

 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

 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

 For each department that has more than 2 professors older than 40, what is the average salary of these professors? SELECT dept,avg(salary) FROM professor WHERE (age>40) GROUP BY dept HAVING (COUNT(*)>2);

 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

 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;

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

 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

 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));

 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));

 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?

 Who teaches CS112?

 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