Group By:- Group By clauses is used to groups rows based on the distinct values of the specified columns.
The syntax of the Group by clause is:-
Select column1, column2, column3, aggregate_function(expression )
From TableName Where (condition)
Group By Column1, column2, column3
For Example, suppose we have a table named EmpInfo which contains the information about the Id of the employee and the Id of the project and it contains the following data
Table:- EmpInfo
EmployeeId Projectid
--------------------------
1 1
2 2
3 2
4 2
5 3
Now suppose we want to know about the number of employee belonging to each project then we can use the group by clause as given below:-
select Projectid , count(employeeid) as NumberOfEmployee from EmpInfo Group by ProjectId
This query will give us the following result:-
ProjectId NumberOfEmployee
1 1
2 3
3 1
Having Clause :- Having clause is used in conjunction with the group clause by imposing a condition on the group by clause to further filter the records return by the group by clause.
Syntax for Having Clause:-
Select column1, column2, column3, aggregate_function(expression )
From TableName Where (condition)
Group By Column1, column2, column3
having aggregate_function(expression ) operator value
For example, suppose we want to know which project has more than 2 employee and the total no of their respective employees, we can use the following query
select Projectid , count(employeeid) as NumberOfEmployee from EmpInfo Group by ProjectId having count(employeeid) >2
The query will give us the following result:-
ProjectId NumberOfEmployee
2 3
In the above example, the Having clause further filter the result return by the Group By clause by imposing a condition that only those projectid will be shown who has more than 2 employee.
