group by clause in Sql

The oracle group by clause is used in a select statement to gather data across multiple records and group the results by one
or more columns.

Syntax

The syntax for the oracle group by clause is
select expression1,expression2,......expression_n,
aggregate_function(expression)
from tables
where conditions
group by expression1,expression2,......expression_n;

Arguments or parameters
Expression1,expression2,.....expression_n that are not encapsulated within aggregate function and must be
included in the group by clause.

aggregate_function can be a function such as sum,count,min,max or avg function.

Tables are the tables that you wish to retrive records from.
There must be at least one table listed in the from clause.
Example:- Let us look at how we could use the group by clause with the MIN function.

SQL> select department_id,sum(salary) As "Total_salary_department_wise"
  2  from employees
  3  group by department_id;

DEPARTMENT_ID Total_salary_department_wise
------------- ----------------------------
          100                        51600
           30                        24900
         1000                         7000
          999                         9600
           20                        19000
           70                        10000
           90                        58000
          110                        20300
           50                       156400
           40                         6500
           80                       294900

DEPARTMENT_ID Total_salary_department_wise
------------- ----------------------------
           10                         4400
           60                        28800

13 rows selected.