What is an aggregate function?

w  What is an aggregate function?

   An aggregate function summarizes the results of an expression over a number of rows, returning a single value. The general syntax for most of the aggregate functions is as follows: aggregate_function ([DISTINCT|ALL] expression)

Some of the commonly used aggregate functions are :

•          SUM   COUNT  AVG  MIN  MAX

Consider the following Employee table:

EMPLOYEE ( EMP_ID, NAME, DEPT_NAME, SALARY)

CREATE TABLE EMPLOYEE

(  EMP_ID NUMBER,   NAME VARCHAR2(50),   DEPT_NAME VARCHAR2(50),  SALARY NUMBER);

After the insert when we query the Employee table we get the following results:

Select * from Employee;

INSERT INTO EMPLOYEE VALUES (100,'ABC','ENG',50000);

INSERT INTO EMPLOYEE VALUES (101,'DEF','ENG',60000);

INSERT INTO EMPLOYEE VALUES (102,'GHI','PS',50000);

INSERT INTO EMPLOYEE VALUES (103,'JKL','PS',70000);

INSERT INTO EMPLOYEE VALUES (104,'MNO','SALES',75000);

INSERT INTO EMPLOYEE VALUES (105,'PQR','MKTG',70000);

INSERT INTO EMPLOYEE VALUES (106,‘STU','SALES',null);

COMMIT;

Query 1: To find the sum of all salaries in the organization:

SELECT SUM(SALARY) FROM EMPLOYEE;

375000

Query 2: To find the sum of the salaries grouped by dept

SELECT SUM(SALARY) FROM EMPLOYEE GROUP BY DEPT_NAME

If we take a look at the previous query the information won’t

tell us what’s the sum for a particular department. So to include that

information we add DEPT_NAME in the SELECT

SELECT DEPT_NAME,SUM(SALARY) FROM EMPLOYEE

GROUP BY DEPT_NAME;

The query in the previous slide lists the information for all the

departments. What if we want the information to be restricted only

for a particular department like Engg

Is this query correct?

SELECT DEPT_NAME,SUM(SALARY) FROM EMPLOYEE

GROUP BY

DEPT_NAME

WHERE DEPT_NAME = 'ENG';

No, the query would result in the sql error (in Oracle)

ORA-00933: SQL Command not properly ended

Remember : If we use the aggregate functions then you cannot use

the WHERE clause. In order to get the result what we need to use is

the HAVING clause. So the query would be

SELECT DEPT_NAME,SUM(SALARY) FROM EMPLOYEE

GROUP BY

DEPT_NAME

HAVING DEPT_NAME = 'ENG';

AVG Function

Query 1: If we want to calculate the AVG of all the salaries in

the organization the SQL would be

SELECT AVG(SALARY) FROM EMPLOYEE

62,500

Is this what we expect????

Employee table has 7 records and the salaries are

50,000+60,000+50,000+70,000+75,000+70,000+null/7 = 53571

But we obtained 62500 from the query? Why is this so????

Remember : COUNT(*) is the only function which won’t ignore

Nulls. Other functions like SUM,AVG,MIN,MAX they ignore

Nulls. What it means is in the previous query the salary value for

a particular employee was NULL. So the query

SELECT AVG(SALARY) FROM EMPLOYEE

would ignore nulls and the way the average is calculated then would be

50,000+60,000+50,000+70,000+75,000+70,000/6 = 62500

From the information given in the previous slide what do you think would be the output of the following query

Select COUNT(*),COUNT(SALARY) FROM EMPLOYEE;

It would be

COUNT(*)  COUNT(SALARY)

         7                       6

Because COUNT(*) is not going to ignore the Nulls in the result

whereas COUNT(SALARY) is going to ignore the Nulls.

SELECT student_name,avg(mark) FROM student,enrolment

WHERE student.student_id=enrolment.student_id;

Which one of the following is correct for the query?

(a) The query is not legal

(b) The query retrieves for each student enrolled,his/her name and their

 average mark

(c) The query retrieves for each student enrolled, his/her name and the class average mark

(d) The query retrieves for each student enrolled, his/her name and the mark in each subject

  Is the answer (a) or (b)??????

Query 1: To find the minimum salary within a particular department

SELECT MIN(SALARY),NAME FROM EMPLOYEE

GROUP BY NAME;

Query 2: To find the maximum salary within a particular department

SELECT MAX(SALARY),NAME FROM EMPLOYEE

GROUP BY NAME;