CTE command In Sql Server

Common table expression (CTE):- Common table expression or CTE can be described as the temporary, named record set return by the execution of the query (insert, delete, update, select or update view statement). It is not stored as an object in the database and it last only till the execution of the SQL Query.
It also can reference itself. It can be referenced multiple times. The SQL Scripts of creation of all the tables and their insert data statement is given below:-

Query for creation of the database used in this article and then using it.

Create database CTE_Learning

Query to used the database created above

Use CTE_Learning

SQL Query for creation of the table Employee

Create table Employee (Employeeid int identity(1,1), EmpFname nvarchar(150), EmpLname nvarchar(150), Managerid int, regdate datetime)

SQL Query for creation of the table Project

Create table Project (Projectid int identity(1,1), Projectname nvarchar(150))

SQL Query for creation of the table Empproj

Create table Empproj (Empprojid int identity(1,1), Employeeid int, Projectid int)

Query for inserting the data in the table Employee

Insert into Employee(EmpFname, EmpLname, Managerid, regdate)
    Select ‘Samir’, ‘Sethi’, 0, getdate()
    union all
    Select ‘Vivek’, ‘Johari’, 1, getdate()
    union all
    Select ‘Avinash’, ‘Dubey’, 2, getdate()
    union all
    Select ‘Sunny’, ‘Rana’, 3, getdate()
    union all
    Select ‘Himanshu’,’Aggarwal’,3,getdate()

Query for inserting the data in the table Project 

Insert into Project (Projectname)
            Select ‘Project1’
            Union all
            Select ‘Project2’
            union all
            Select  ‘Project3’
Query for inserting the data in the table Empproj 

Insert into Empproj (Employeeid, Projectid)
               Select 1,1
              Union  all
               select 1, 2
              Union  all
               select 1,3
              Union  all
               select 2,1
              Union  all
               Select 2,2
              Union  all
                Select 3,1
              Union all
                Select 4,2
 The Common table expression has the following syntax:

With Expressionname (Column list)
AS
(
   CTE query definition
)

Here, Expressionname is the name of the CTE, Column list is the name of the unique column names which are return as the record set through the execution of the CTE query and CTE query definition is the SQL query which we are going to use in the CTE.

Suppose, we want to get the recordset which contains the details of the employees who are attacted to more than 1 projects then we can use the CTE as given below:-

With CTE_Projinfo (employeeid, total_projects) 
As
(
  select employeeid, count(employeeid) from empproj group by employeeid having count(employeeid)>1
)

select e.employeeid , e.empfname + ‘ ‘ + e.empLname, e.regdate from employee e inner join CTE_Projinfo
on e.employeeid = CTE_Projinfo.employeeid

Here CTE_Projinfo is the CTE expression name
        employeeid, total_projects are the names of the columns return by the execution of the CTE query

select employeeid, count(employeeid) from empproj group by employeeid having count(employeeid)>1

In the above example, we have used the CTE “CTE_Projinfo ” to get the details of the employees by using the inner join with the table employee.

Also we can includes multiple CTEs within a WITH Clause
For example,

With
    CTE_Projinfo (employeeid, total_projects) 
As
(
  select employeeid, count(employeeid) from empproj group by employeeid having count(employeeid)>1
),

CTE_Empproject (employeeid, projectname)
AS
(
  Select CTE_Projinfo.employeeid,Projectname from CTE_Projinfo  inner join  Empproj
on CTE_Projinfo.Employeeid=Empproj.employeeid inner join Project 
on Empproj.projectid=Project.projectid
)

select e.employeeid , e.empfname + ‘ ‘ + e.empLname, e.regdate,CTE_Empproject.projectname from employee e inner join CTE_Empproject 
on e.employeeid =  CTE_Empproject.employeeid 

  
Since a CTE can reference itself also, we can use the CTE to get the hierarchy of an organization. 

For example, we can use the following CTE to get the  hierarchical data of the employees of an organisation.
      
With CTE_managerlevel (Employeeid,employeename,managerid,joblevel) as
(
  select e.employeeid , e.empfname + ‘ ‘ + e.empLname, e.Managerid,0 from employee e where e.employeeid=1
  UNION ALL
 select m.employeeid , m.empfname + ‘ ‘ + m.empLname, m.Managerid,CTE_managerlevel.joblevel+1 from employee m inner join CTE_managerlevel
 on m.Managerid=CTE_managerlevel.employeeid
)

select Employeeid,employeename,managerid,joblevel from CTE_managerlevel