RELATIONAL DATABASE RELATIONAL DATABASE MANAGEMENT SYSTEMMANAGEMENT SYSTEM

PRESENTED TO:Prof. kavita Prof. kavita

SainiSaini

PRESENTED BY:MUKESH PANDEY

GRADUATE SCHOOL OF

BUSINESS&ADMN,GREATER NOIDA

Aggregate Functions

What is an aggregate function? An aggregate function summarizes the results

of an expression over a number of rows, returning a single value.

Aggregate functions used

Some of the commonly used aggregate functionsare :• SUM• COUNT• AVG• MIN• MAX

Examples

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);

Employee Table (Contd….)

Run the following script to insert the records in the table

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;

Select on Employee Table

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

Select * from Employee;

Performing SUM

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 deptSELECT SUM(SALARY) FROM EMPLOYEE GROUP BYDEPT_NAME

SUM (Continued)

If we take a look at the previous query the information won’ttell 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;

SUM (Continued…..)

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 BYDEPT_NAME WHERE DEPT_NAME = 'ENG';

SUM (Continued….)

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 usethe 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 BYDEPT_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 EMPLOYEE62,500

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?

AVG (Continued….)

Remember : COUNT(*) is the only function which won’t ignoreNulls. Other functions like SUM,AVG,MIN,MAX they ignore Nulls. What it means is in the previous query the salary value fora particular employee was NULL. So the query

SELECT AVG(SALARY) FROM EMPLOYEEwould ignore nulls and the way the average is calculated then wouldbe 50,000+60,000+50,000+70,000+75,000+70,000/6 = 62500

AVG (Continued….)

From the information given in the previous slide what do you thinkwould 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.

Using MIN AND MAX

Query 1: To find the minimum salary within a particular departmentSELECT MIN(SALARY),NAME FROM EMPLOYEEGROUP BY NAME;

Query 2: To find the maximum salary within a particular departmentSELECT MAX(SALARY),NAME FROM EMPLOYEEGROUP BY NAME;

Count functionCount function

The COUNT() function returns the number of rows that matches a specified criteria.

The COUNT(column_name) function returns the number of values (NULL values will not be counted) of the specified column:

SQL COUNT Syntax: SELECT COUNT(column_name) FROM

table_name

We have the following "Orders" table

O_Id OrderDate OrderPrice Customer

1 2008/11/12 1000 Hansen

2 2008/10/23 1600 Nilsen

3 2008/09/02 700 Hansen

4 2008/09/03 300 Hansen

5 2008/08/30 2000 Jensen

6 2008/10/04 100 Nilsen

Performing count

Now we want to count the number of orders from "Customer Nilsen".

We use the following command:-

SELECT COUNT(Customer) AS CustomerNilsen FROM OrdersWHERE Customer='Nilsen'

CustomerNilsen

2

The result of the SQL statement above will be 2, because the customer Nilsen has made 2 orders in total:

GROUP BY clauseGROUP BY clause

Aggregate functions often need an added GROUP BY statement.

The GROUP BY Statement The GROUP BY statement is used in conjunction with the

aggregate functions to group the result-set by one or more columns.

SQL GROUP BY Syntax SELECT column_name,

aggregate_function(column_name)FROM table_name GROUP BY column_name

Performing group by

O_Id OrderDate OrderPrice Customer1 2008/11/12 1000 Hansen2 2008/10/23 1600 Nilsen3 2008/09/02 700 Hansen4 2008/09/03 300 Hansen5 2008/08/30 2000 Jensen6 2008/10/04 100 Nilsen

We have the following "Orders" table:

SELECT Customer,SUM(OrderPrice) FROM Orders GROUP BY Customer

Now we want to find the total sum (total order) of each customer.We will have to use the GROUP BY statement to group the customers.We use the following command:

Customer SUM(OrderPrice)

Hansen 2000

Nilsen 1700

Jensen 2000

The result-set will look like this:

Having clauseHaving clause

The HAVING clause was added to SQL because the WHERE keyword could not be used with aggregate functions.

SQL HAVING Syntax SELECT column_name,

aggregate_function(column_name)FROM table_name GROUP BY column_nameHAVING aggregate_function(column_name)

Performing having

SELECT Customer,SUM(OrderPrice) FROM OrdersGROUP BY CustomerHAVING SUM(OrderPrice)<2000

Customer SUM(OrderPrice)Nilsen 1700

Now we want to find if any of the customers have a total order of less than 2000.We use the following command:

The result-set will look like this: