Derived Table In Sql Server

Derived tables:-  Derived tables are the tables which are created on the fly with the help of the Select statement. It is different from the temporary table in the way that  in case of temporary table, first we have to create a temporary table,  insert the data into the table, select the data from the temporary table and then we have to drop the temporary table. But in case of derived table, SQL Server itself create and populate the table in the memory and we can directly use it. Also we don,t need to drop it. But it can only be referenced  by the outer Select query who created it. Also since it is reside in the memory itself, it is faster then Temporary tables which are created in the temp database.Suppose, there is a table say tbl_studentcoursereg to store the id of the students registered for various courses, registration month and date. Table structure is given below:-

create table tbl_studentcoursereg (id int identity(1,1),  studentid int, coursename nvarchar(100), Regmonth nvarchar(50), regdate datetime)

The insert command to populate this table with the data is given below:-

insert into tbl_studentcoursereg
values (1, ‘C#’, ‘JAN’,’01/01/2012′)

insert into tbl_studentcoursereg
values (2, ‘SQL’, ‘JAN’,’01/02/2012′)

insert into tbl_studentcoursereg
values (3, ‘C++’, ‘JAN’,’01/03/2012′)

insert into tbl_studentcoursereg
values (4, ‘C#’, ‘FEB’,’02/02/2012′)

insert into tbl_studentcoursereg
values (5, ‘C#’, ‘MAR’,’03/03/2012′)

insert into tbl_studentcoursereg
values (6, ‘JAVA’, ‘APR’,’04/04/2012′)

insert into tbl_studentcoursereg
values (7, ‘JAVA’, ‘APR’,’04/04/2012′)

insert into tbl_studentcoursereg
values (8, ‘ORACLE’, ‘APR’,’04/04/2012′)

insert into tbl_studentcoursereg
values (9, ‘C#’, ‘APR’,’04/23/2012′)

insert into tbl_studentcoursereg
values (10, ‘C#’, ‘MAY’,’05/05/2012′)

Now if we want to know in which month number of students registered for a particular months is greater than 2, the we can use the following query which uses the derived table.

select regmonth, totalstud, totalcourse from
(select  regmonth ,count(studentid) as totalstud ,
count(coursename)as totalcourse from tbl_studentcoursereg group by regmonth )as tbl1  where tbl1.totalstud>2