6. Aggregate Functions.pdf
- 1. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 1 6. Aggregate Functions Aggregate Function Aggregate functions are functions that take a collection (a set or multiset) of values as input and return a single value. SQL offers five built-in aggregate functions: Average: avg Minimum: min Maximum: max Total: sum Count: count The input to sum and avg must be a collection of numbers, but the other operators can operate on collections of nonnumeric data types, such as strings, as well. Find the average salary of instructors in the Computer Science department. SQL> select * from instructor; ID NAME DEPT_NAME SALARY ----- -------------------- -------------------- ---------- 10101 Srinivasan Comp.Sci. 65000 12121 Wu Finance 90000 15151 Mozart Music 40000 22222 Einstein Physics 95000 32343 El Said History 60000 33456 Gold Physics 87000 45565 Katz Comp.Sci. 75000 58583 Califieri History 62000 76543 Singh Finance 80000 76766 Crick Biology 72000 83821 Brandt Comp.Sci. 92000 ID NAME DEPT_NAME SALARY ----- -------------------- -------------------- ---------- 98345 Kim Elec.Eng. 80000 12 rows selected. SQL> select avg (salary) 2 from instructor 3 where dept_name= 'Comp.Sci.'; AVG(SALARY) -----------
- 2. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 2 77333.3333 Give a meaningful name to the attribute by using the as clause SQL> select * from instructor; ID NAME DEPT_NAME SALARY ----- -------------------- -------------------- ---------- 10101 Srinivasan Comp.Sci. 65000 12121 Wu Finance 90000 15151 Mozart Music 40000 22222 Einstein Physics 95000 32343 El Said History 60000 33456 Gold Physics 87000 45565 Katz Comp.Sci. 75000 58583 Califieri History 62000 76543 Singh Finance 80000 76766 Crick Biology 72000 83821 Brandt Comp.Sci. 92000 ID NAME DEPT_NAME SALARY ----- -------------------- -------------------- ---------- 98345 Kim Elec.Eng. 80000 12 rows selected. SQL> select avg (salary) as avg_salary 2 from instructor 3 where dept_name='Comp.Sci.'; AVG_SALARY ---------- 77333.3333 Find the total number of instructors who teach a course in the spring 2010 semester SQL> select * from teaches; ID COURSE_I SEC_ID SEMEST YEAR ----- -------- -------- ------ ---------- 10101 CS-101 1 Fall 2009 10101 CS-315 1 Spring 2010 10101 CS-347 1 Fall 2009 12121 FIN-201 1 Spring 2010
- 3. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 3 15151 MU-199 1 Spring 2010 22222 PHY-101 1 Fall 2009 32343 HIS-351 1 Spring 2010 45565 CS-101 1 Spring 2010 45565 CS-319 1 Spring 2010 76766 BIO-101 1 Summer 2009 76766 BIO-301 1 Summer 2010 ID COURSE_I SEC_ID SEMEST YEAR ----- -------- -------- ------ ---------- 83821 CS-190 1 Spring 2009 83821 CS-190 2 Spring 2009 83821 CS-319 2 Spring 2010 98345 EE-181 1 Spring 2009 15 rows selected. SQL> select count (distinct ID) 2 from teaches 3 where semester = 'Spring' and year = 2010; COUNT(DISTINCTID) ----------------- 6 Find the number of tuples in the course relation SQL> select * from course; COURSE_ TITLE DEPT_NAME ------- -------------------------------------------------- -------------------- CREDITS ---------- BIO-101 Intro. to Biology Biology 4 BIO-301 Genetics Biology 4 BIO-399 Computational Biology Biology 3 COURSE_ TITLE DEPT_NAME ------- -------------------------------------------------- --------------------
- 4. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 4 CREDITS ---------- CS-101 Intro. to Computer Science Comp.Sci. 4 CS-190 Game Design Comp.Sci. 4 CS-315 Robotics Comp.Sci. 3 COURSE_ TITLE DEPT_NAME ------- -------------------------------------------------- -------------------- CREDITS ---------- CS-319 Image Processing Comp.Sci. 3 CS-347 Database System Concepts Comp.Sci. 3 EE-181 Intro. to Digital Systems Elec.Eng. 3 COURSE_ TITLE DEPT_NAME ------- -------------------------------------------------- -------------------- CREDITS ---------- FIN-201 Investment Banking Finance 3 HIS-351 World History History 3 MU-199 Music Video Production Music 3 COURSE_ TITLE DEPT_NAME ------- -------------------------------------------------- -------------------- CREDITS ---------- PHY-101 Physical Principles Physics 4
- 5. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 5 13 rows selected. SQL> select count (*) 2 from course; COUNT(*) ---------- 13
- 6. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 6 GROUP BY Clause There are circumstances where we would like to apply the aggregate function not only to a single set of tuples, but also to a group of sets of tuples; we specify this wish in SQL using the group by clause. The attribute or attributes given in the group by clause are used to form groups. Tuples with the same value on all attributes in the group by clause are placed in one group. Find the average salary in each department. SQL> select * from instructor; ID NAME DEPT_NAME SALARY ----- -------------------- -------------------- ---------- 10101 Srinivasan Comp.Sci. 65000 12121 Wu Finance 90000 15151 Mozart Music 40000 22222 Einstein Physics 95000 32343 El Said History 60000 33456 Gold Physics 87000 45565 Katz Comp.Sci. 75000 58583 Califieri History 62000 76543 Singh Finance 80000 76766 Crick Biology 72000 83821 Brandt Comp.Sci. 92000 ID NAME DEPT_NAME SALARY ----- -------------------- -------------------- ---------- 98345 Kim Elec.Eng. 80000 12 rows selected. SQL> select dept_name, avg (salary) as avg_salary 2 from instructor 3 group by dept_name; DEPT_NAME AVG_SALARY -------------------- ---------- Physics 91000 Finance 85000 Elec.Eng. 80000 Comp.Sci. 77333.3333 Biology 72000 Music 40000 History 61000
- 7. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 7 7 rows selected. Find the number of instructors in each department who teach a course in the spring 2010 semester. SQL> select * from instructor; ID NAME DEPT_NAME SALARY ----- -------------------- -------------------- ---------- 10101 Srinivasan Comp.Sci. 65000 12121 Wu Finance 90000 15151 Mozart Music 40000 22222 Einstein Physics 95000 32343 El Said History 60000 33456 Gold Physics 87000 45565 Katz Comp.Sci. 75000 58583 Califieri History 62000 76543 Singh Finance 80000 76766 Crick Biology 72000 83821 Brandt Comp.Sci. 92000 ID NAME DEPT_NAME SALARY ----- -------------------- -------------------- ---------- 98345 Kim Elec.Eng. 80000 12 rows selected. SQL> select * from teaches; ID COURSE_I SEC_ID SEMEST YEAR ----- -------- -------- ------ ---------- 10101 CS-101 1 Fall 2009 10101 CS-315 1 Spring 2010 10101 CS-347 1 Fall 2009 12121 FIN-201 1 Spring 2010 15151 MU-199 1 Spring 2010 22222 PHY-101 1 Fall 2009 32343 HIS-351 1 Spring 2010 45565 CS-101 1 Spring 2010 45565 CS-319 1 Spring 2010 76766 BIO-101 1 Summer 2009 76766 BIO-301 1 Summer 2010 ID COURSE_I SEC_ID SEMEST YEAR ----- -------- -------- ------ ----------
- 8. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 8 83821 CS-190 1 Spring 2009 83821 CS-190 2 Spring 2009 83821 CS-319 2 Spring 2010 98345 EE-181 1 Spring 2009 15 rows selected. SQL> select dept_name, id from instructor natural join teaches; DEPT_NAME ID -------------------- ----- Comp.Sci. 10101 Comp.Sci. 10101 Comp.Sci. 10101 Finance 12121 Music 15151 Physics 22222 History 32343 Comp.Sci. 45565 Comp.Sci. 45565 Biology 76766 Biology 76766 DEPT_NAME ID -------------------- ----- Comp.Sci. 83821 Comp.Sci. 83821 Comp.Sci. 83821 Elec.Eng. 98345 15 rows selected. SQL> select dept_name, count (distinct ID) as instr_count 2 from instructor natural join teaches 3 where semester = 'Spring' and year = 2010 4 group by dept_name; DEPT_NAME INSTR_COUNT -------------------- ----------- Comp.Sci. 3 Finance 1 History 1 Music 1
- 9. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 9 Each instructor in a particular group (defined by dept name) can have a different ID, and since only one tuple is output for each group, there is no unique way of choosing which ID value to output. SQL> select dept_name, ID, avg (salary) 2 from instructor 3 group by dept_name; select dept_name, ID, avg (salary) * ERROR at line 1: ORA-00979: not a GROUP BY expression SQL> select dept_name, avg (salary) 2 from instructor 3 group by dept_name; DEPT_NAME AVG(SALARY) -------------------- ----------- Physics 91000 Finance 85000 Elec.Eng. 80000 Comp.Sci. 77333.3333 Biology 72000 Music 40000 History 61000 7 rows selected.
- 10. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 10 HAVING Clause At times, it is useful to state a condition that applies to groups rather than to tuples. For example, we might be interested in only those departments where the average salary of the instructors is more than $42,000. This condition does not apply to a single tuple; rather, it applies to each group constructed by the group by clause. To express such a query, we use the having clause of SQL. SQL applies predicates in the having clause after groups have been formed, so aggregate functions may be used. Find the departments where the average salary of the instructors is more than $42,000 SQL> select * from instructor; ID NAME DEPT_NAME SALARY ----- -------------------- -------------------- ---------- 10101 Srinivasan Comp.Sci. 65000 12121 Wu Finance 90000 15151 Mozart Music 40000 22222 Einstein Physics 95000 32343 El Said History 60000 33456 Gold Physics 87000 45565 Katz Comp.Sci. 75000 58583 Califieri History 62000 76543 Singh Finance 80000 76766 Crick Biology 72000 83821 Brandt Comp.Sci. 92000 ID NAME DEPT_NAME SALARY ----- -------------------- -------------------- ---------- 98345 Kim Elec.Eng. 80000 12 rows selected. SQL> select dept_name, avg (salary) as avg_salary 2 from instructor 3 group by dept_name 4 having avg (salary) > 42000; DEPT_NAME AVG_SALARY -------------------- ---------- Physics 91000 Finance 85000 Elec.Eng. 80000 Comp.Sci. 77333.3333 Biology 72000
- 11. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 11 History 61000 6 rows selected.
- 12. Mrs. Sunita M Dol, CSE Department WIT, Solapur Page 12 References: Database system concepts by Abraham Silberschatz, Henry F. Korth, S. Sudarshan (McGraw Hill International Edition) sixth edition. Database system concepts by Abraham Silberschatz, Henry F. Korth, S. Sudarshan (McGraw Hill International Edition) fifth edition. http://codex.cs.yale.edu/avi/db-book/db4/slide-dir/ http://codex.cs.yale.edu/avi/db-book/db5/slide-dir/ http://codex.cs.yale.edu/avi/db-book/db6/slide-dir/