Introduction to Oracle Functions and Group By

Clause

Introduction to Oracle Functions

Functions make the result of the queryeasier and are used to manipulate the datavalues. Functions can accept any number ofconstant values or variables. Thesevariables or constant are called asarguments. SQL functions can performdifferent types of operations such asmodify individual data items, convertcolumn data types, format dates andnumbers etc.

Categories of Functions

Oracle functions are categorized into two categories:• Single Row/Scalar Functions• Group/Aggregate Functions

Functions, which operate on single rows and return one value per row, are called as Scalar functions or Single Row Functions. On the other hand functions, which operate on set of values to give one result, are called as Group Functions or Aggregate Functions.

Single-Row Functions (Scalar functions)

These functions act on each row of the table andreturn a single value for each row selected. Thesefunctions may accept one or more arguments and canreturn a value of different data type than that ithas accepted.

Classification of Single Row Functions

Single Row Functions can be classified into the following categories:

(i) Character

(ii) Number

(iii) Date

(iv) Conversion

(v) General

Character Functions

•

• length(x)

It returns the length of the string x.

Example:

SQL> Select LENGTH ('Ipso Facto') ergo FROM dual;

• ltrim(string[,char(s)])

It removes all the blank spaces from the left side of thestring if no char is specified. If we give a char, then itremoves the leading occurrences of that character from thestring.

• Translate(char,find,new)

This function is used to find a char and replace it with newcharacter. All occurrences of find are replaced by the character in new.

• floor(x)Where x is a number. This function returns the largest integer that is less than or equal to n. FLOOR round

down to a whole number.

• round(x[,y])It rounds off x to the decimal precision of y. If y is negative, rounds to the precision of y places to the left of the

decimal point.

• sqrt(x)This function returns the square root of the given number x. If the given number x is negative or NULL then the result is

NULL.Example:SQL>select sqrt(36) as square_root from dual;

Date Functions

SQL> SELECT TO_CHAR(SYSDATE,'HH') HOUR, TO_CHAR(SYSDATE,'MI') MIN,TO_CHAR(SYSDATE,'SS') SEC FROM DUAL;

The output is:

HO MI SE

-- -- --

03 01 16

SQL> SELECT TO_DATE('15-MAR-1999','DD-MON-YYYY')FROM DUAL;

SQL>SELECT TO_NUMBER('49583') FROM DUAL;

General functions

SQL>Select greatest(-2 ,10,’7’) from dual ;

SQL>Select least(‘ABCD’,‘abcd’,’xyz’)from dual ;

SQL>Select user from dual;

Aggregate Functions (Group Functions)

These functions are used to produce summarized results. They are applied on set of rows to give you single value as a result. A group function allows you to perform a data operation on several values in a column of data as though the column was one collective group of data. These functions are called group functions also, because they are often used in a special clause of select statements called a group by clause.

COUNT (x)

This function returns the number of rows or non-null values for column x. When we use * in place of x, it returns the total number of rows in the table.

Syntax:count([distinct|all]column name)Example:1. Count the number of employees in the emp table.

SQL>Select count(empno) from emp;

The Output is:COUNT(EMPNO)------------16

List the number of different names in the emp table.

SQL>Select count (distinct ename) from emp;

The output is:COUNT(DISTINCTENAME)--------------------

16List the number of departments in the employee table .

SQL>Select count( distinct deptno) from emp;

The output is:COUNT(DISTINCTDEPTNO)---------------------

3

SUM(x)

This function returns the sum of values for the column x. This function is applied on columns having numeric datatype and it returns the numeric value.

syntax : sum([distinct|all]column name)

Example:

List the total salary paid to the employees in the emp table.

SQL>select sum(sal) from emp ;The output is:SUM(SAL)---------

29025

AVG(x)

This function returns the average of values for the column x. This function is applied on columns having numeric datatype and it returns the numeric value. It ignores the null values in the column x.

syntax : avg([distinct|all]column name)

Example:

List the average salary and the number of employees in the emp table .

SQL>select avg(sal) ,count(sal) from emp ;

MIN(x)

This function returns the minimum of values for the column x for all the rows .this function can be applied on any datatype .

syntax : min([distinct|all]column name)

Example:List the minimum salary in the emp table .

SQL>select min(sal) from emp ;The output is:MIN(SAL)--------

800

MAX(x)This function returns the maximum of values for

the column x for all the rows .this function can be applied on any datatype.

syntax : max([distinct|all]column name)

Example:List the maximum salary and commission in the emp table .

SQL>select max(sal) ,max(comm) from emp ;

Note : The avg() and sum() functions will always be applied on numeric datatype while min() and max() functions can be applied on any datatype.

Example

SQL>select avg(sal),sum(sal),min(ename),max(ename) from emp ;

Exercise :

•list the names of the employees earning minimum salary .•list the names of the employees earning second highest salary .•list the details of the employees who earn salary greater than the average salary . also count their number .•count the number of employees whose salary is equal to the highest salary .•list the number of employees ,their average salary ,minimum salary and maximum salary in the employee table.

Grouping Data with GROUP

BYGROUP BY clause is used to group or categorize the data. In other

words it divide rows in a table into smaller groups. We can then

use the group functions to return summary information for each

group.

If no GROUP BY clause is specified, then the default grouping is

the entire result set. When the query executes and the data is

fetched, it is grouped based on the GROUP BY clause and the

group function is applied.

Syntax:

SELECT column,group_function(column) FROM table

[WHERE condition]

[GROUP BY group_by_expression]

[ORDER BY column];

Here, group_by_expression specifies columns whose values

determine the basis for grouping rows.

For example, If we have to find the total salary of each department

manually, first we group the records on the basis of department

number and then we apply the sum function on salary of each group

to obtain the required result. Similarly in SQL we apply the GROUP

BY clause on deptno and then calculate the total salary for each group

by Sum(sal) function as shown below:

SQL>SELECT deptno, Sum(sal) FROM emp GROUP BY

deptno;

The output is:

DEPTNO SUM(SAL)

10 2916.6667

20 2175

30 1566.6667

Here is how this SELECT statement, containing a GROUP BY clause,

is evaluated:

• The SELECT clause specifies the columns to be retrieved i.e

Department number column in the EMP table, the sum of all the

salaries in the group you specified in the GROUP By clause

• The FROM clause specifies the tables that the database must

access i.e EMP table.

• The WHERE clause specifies the rows to be retrieved. Since

there is no WHRE clause, by default all rows are retrieved.

The GROUP BY clause specifies how the rows should be grouped.

Department number groups the rows, so the AVG function that is

being applied to the salary column will calculate the average salary

for each department.

• List the average salary of each job in the emp table.

SQL>SELECT JOB,AVG(SAL) FROM EMP GROUP BY JOB;

•

List the maximum salary for each dept.

SQL>SELECT DEPTNO,MAX(SAL) FROM EMP GROUP BY

DEPTNO;

Grouping by more than one

column

Sometimes there is a need to see results for groups within groups.

For example if we have to find the total salary being paid to each

job title, within each department. Then there is a need to having

grouping on department number and within each department

number grouping on the basis of job or in other words there is a

need for grouping within a group.

Thus, the EMP table is grouped first by department number, and

within that grouping, it is grouped by job title. For example, the two

clerks in department 20 are grouped together and a single result

(total salary) is produced for all clerks people within that group.

SQL> SELECT deptno,job,sum(sal) FROM emp GROUP BY

deptno, job;

The output is:

By above example it is clear that we can return summary results for

groups and subgroups by listing more than one GROUP BY column.

We can determine the default sort order of the results by the order of

the columns in the GROUP BY clause.

The SELECT clause specifies the column to be retrieved:

• Department number in the EMP table

• Job title in the EMP table

• The sum of all the salaries in the group that you specified in the

GROUP BY clause

• The FROM clause specifies the tables that the database must

access the EMP table

• The GROUP BY clause specifies how we must group the rows

First, department number groups the rows. Second, within the

department number groups, the rows are grouped by job title. So, the

SUM function is being applied to the salary column for all job titles

within each department number group.

Illegal Queries Using Group

Functions

Whenever you use a mixture of individual items (DEPTNO) and

group functions (COUNT) in the same select statement, you must

include a Group By clause that specifies the individual items (in

this case, DEPTNO). If the GROUP By clause is missing, then the

error message “not a single-group group function” appears and an

asterisk (*) points to the offending column. You can correct the

error by adding the GROUP BY clause.

For example, following is the illegal query:

SQL> SELECT deptno, COUNT(ename) FROM emp;

The Output will be:

Column missing in the Group By clause

Select deptno,count(ename)

*

ERROR at line 1:

ORA-00937: not a single-group group function In above select

statement individual items DEPTNO and group function COUNT

appears in the same SELECT statement without GROUP BY clause

which results error, it can be corrected by adding the GROUP BY

clause as shown below:

SQL> SELECT deptno,COUNT(ename) FROM emp Group By deptno;

DEPTNO COUNT(ENAME)

10 3

20 5

30 6

Note:

Any column or expression in the SELECT list that is not an

aggregate function must be in the GROUP By clause.

Restricting Group Results

As we use the WHERE clause to restrict the rows that we select,

we can use the HAVING clause to restrict groups.

For example: To find the maximum salary of each department,

but show only the departments that have a maximum salary of

more than Rs.2900, we need to do the following.

• Find the maximum salary for each department by grouping

by department number.

• Restrict the groups to those departments with a maximum

salary greater the Rs.2900.

Syntax:

SELECT column, group_function

FROM table

{WHERE condition]

[GROUP BY group_by_expression]

[HAVING group_condition]

[ORDER BY Column];

Here we use the HAVING clause to specify which groups are to be

displayed. Therefore, we further restrict the groups on the basis of

aggregate information.

In the syntax:

HAVING clause restricts the groups of rows returned to those

groups for which the specified condition is TRUE

The Oracle Server performs the following steps when you use the

HAVING clause:

• Rows are grouped

• The group function is applied to the group.

• The groups that match the criteria in the HAVING clause are

displayed.

The HAVING clause can precede the GROUP By clause, but it is

recommended that you place the GROUP By clause first because it is

more logical. Groups are formed and group functions are calculated

before the HAVING clause is applied to the groups in the SELECT

list.

For example: To find the maximum salary of each department, but

show only the departments that have a maximum salary of more than

Rs.2900

SQL> SELECT deptno,max(sal) FROM emp

GROUP BY deptno Having max(sal)>2900;

The output is:

DEPTNO MAX(SAL)

10 5000

20 3000

Use of WHERE clause with GROUP BY clause

List the total salary, maximum and minimum salary and the average

salary of employees job wise, for department number 20 and

display only those rows having average salary greater than 1000

SQL>SELECT job, SUM(sal), avg(sal), max(sal), min(sal) from

emp

WHERE deptno=20

GROUP by job

HAVING AVG(sal)>1000;

The output is:

JOB SUM(SAL) AVG(SAL) MAX(SAL) MIN(SAL)

ANALYST 6000 3000 3000 3000

MANAGER 2975 2975 2975 2975

SQL> SELECT job, SUM(sal) PAYROLL

FROM emp

WHERE job NOT LIKE 'SALE%'

GROUP BY job

HAVING SUM(sal)>5000

ORDER BY SUM(sal);

The output is:

JOB PAYROLL

ANALYST 6000

MANAGER 8275

The above query displays the job title and total monthly salary for

each job title with a total payroll exceeding Rs.5000. The example

excludes salespeople and sorts the list by the total monthly salary.

Display total no of suppliers supplying red part

Display total qty supplied by by each supplier

Display total Qty supplied for each part excluding P3

Only display those where supplied qty is greater than100

Display info in descending order of Qty

Select Pno, Sum(QTY) from P,SP WHERE PNO<>’P3’

GROUP BY PNO HAVING SUM(QTY)>1000 ORDER BY

PNO;

Display total Qty supplied for each part excluding part having red

color, Only display those where supplied qty is greater than100

Select Pno, Sum(QTY) from P,SP WHERE PNO NOT

IN(SELECT PNO FROM P WHERE COLOR=‘RED’) GROUP

BY PNO HAVING SUM(QTY)>1000 ORDER BY PNO;