Group By Clause

Group By Clause

Group by clause is a clause of select statement responsible to logically segregate the table data in different blocks. This clause is only meant for multirow function. In this topic, we will discuss one of the major clauses in SQL that is the group by clause in DQL(Data Query Language).

Please read the previous post
Introduction to Structured Query Language (SQL)
Object-Relational Database
SELECT Statement in SQL

This clause is only meant for multirow function.

Select <exp>
[group by<col_name>[,col_name,…………..n]];

Now we get more idea on group clause based on the below example

Q. Write a query which will display department wise sum(sal), max(sal), min(sal) ?
Select sum(sal), max(sal), min(sal)
From emp
Group by deptno;


Only that column is allowed in select list, the column which is used in group by clause.

Select ename, sum(sal), max(sal),min(sal)
From emp
Group by deptno;   

The above line of SQL statement will get error because ename (i.e. Employee Name) is not directly depend up on department group.

Group By Clause for multiple column

Now we will working with multiple column wise grouping:

Q. Write a query which will display deptwise , job wise  sum(sal),max(sal),min(Sal) ?
Select deptno, job , sum(sal),max(sal), min(sal) from emp
Group by deptno, job; 

Count() function in SQL

Count is a multirow() responsible to count the number of values of a column or no. of records of a group. If can accept two types of arguments in function is *, col_name.

Select count(*) from emp;
--based on column name 
Select count(mgr) from emp ;
Q. Write query  which will display department wise , job wise, sum(sal),max(sal), min(Sal) and the no. of employee  working in each group.
Select deptno, max(sal),min(sal),sum(sal), count(*) from emp
Group by deptno,job;

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