SQL Subqueries and Basic Operations

1. Chapter 3Introduction to SQL(4) 1st Semester, 2017 Sanghyun Park

2. Outline • History • Data Definition • Basic Query Structure • Additional Basic Operations • Set Operations • Null Values • Aggregate Functions • Nested Subqueries • Database Modification

3. Nested Subqueries • SQL provides a mechanism for the nesting of subqueries • A subquery is a select-from-where expression that is nested within another query • The nesting can be done in the following SQL query select A1, A2, …, Anfrom r1, r2, .., rmwhere P • Ai can be replaced by a subquery that generates a single value • ri can be replaced by any valid subquery • P can be replaced with an expression of the form: B <operation> (subquery)where B is an attribute and <operation> will be defined later

4. Subqueries in Where Clause • A common use of subqueries is to perform tests: • For set membership • For set comparison • For set cardinality

5. Set Membership • Find courses offered both in Fall 2009 and in Spring 2010selectdistinct course_idfrom sectionwhere semester = ‘Fall’ and year = 2009 and course_id in (select course_id from section where semester = ‘Spring’ and year = 2010); • Find courses offered in Fall 2009 but not in Spring 2010selectdistinct course_idfrom sectionwhere semester = ‘Fall’ and year = 2009 and course_id not in (select course_id from section where semester = ‘Spring’ and year = 2010);

6. Set Comparison – “some” Clause • Find names of instructors with salary greater than that of some (at least one) instructor in the Biology departmentselectdistinct T.namefrom instructor as T, instructor as Swhere T.salary > S.salary and S.dept_name = ‘Biology’ • Same query using > some clauseselect namefrom instructorwhere salary > some (select salary from instructor where dept_name = ‘Biology’);

7. Set Comparison – “all” Clause • Find names of instructors whose salary is greater than the salary of all instructors in the Biology departmentselect namefrom instructorwhere salary > all (select salary from instructor where dept_name = ‘Biology’);

8. Test for Empty Relations • The exists construct returns the value true if the argument subquery is nonempty • exists r  r ≠ Ø • not exists r  r = Ø

9. Use of “exists” Clause • Yet another way of specifying the query “Find all courses taught both in Fall 2009 and in Spring 2010”select course_idfrom section as Swhere semester = ‘Fall’ and year = 2009 andexists (select * from section as T where semester = ‘Spring’ and year = 2010and S.course_id = T.course_id);

10. Use of “not exists” Clause • Find all students who have taken all courses offered in the Biology department selectdistinct S.ID, S.namefrom student as Swherenotexists ( (select course_id from course where dept_name = ‘Biology’ )except (select T.course_idfrom takes as Twhere S.ID = T.ID ) );

11. Test for Absence of Duplicate Tuples • The unique construct tests whether a subquery has any duplicate tuples in its result • The unique construct evaluates to “true” if a given subquery contains no duplicates • Find all courses that were offered at most once in 2009 select T.course_idfrom course as Twhereunique (select R.course_id from section as R where T.course_id = R.course_id and R.year = 2009 );

12. Subqueries in From Clause • 생략 (textbook 참조)

13. Subqueries in Select Clause • 생략 (textbook 참조)

14. Database Modification - Delete • Delete all instructors from the Finance dept:delete from instructorwheredept_name = ‘Finance’ • Delete all instructors whose salary is less than the average salary of instructors:delete from instructorwhere salary< (select avg (salary)from instructor)

15. Database Modification - Insert • Add a new tuple to course: insert into coursevalues (‘CS-437’, ‘Database Systems’, ‘Comp. Sci’, 4) or equivalentlyinsert into course(course_id, title, credits, dept_name)values (‘CS-437’, ‘Database Systems’, 4, ‘Comp. Sci’) • Add all instructors to the student relation with tot_creds set to 0:insert into studentselect ID, name, dept_name, 0from instructor

16. Database Modification - Update • Increase salaries of instructors whose salary is over $100,000 by 3% and all others receive 5% raise: update instructorset salary = salary  1.03where salary > 100000; update instructorset salary = salary  1.05where salary  100000; • The order is important • Can be done better using the case statementupdate instructorset salary =casewhen salary  100000 then salary *1.05else salary * 1.03end;