CTE (Common Table Expression) In Depth

CTE (common table expression) and Updatable CTE

CTE was introduced in the 2005 SQL Server. CTE is like a temporary result set which is defined within the execution of the current context or execution scope of a single select, insert, update delete and/or create view statement.

 

It is similar to a derived table and it is not stored as an object like other objects in the SQL server.

 

Remember -- CTE table is created with the keyword.  

with CTEtable
    as
    (
    select d.Department_Name as deptname, COUNT(e.empid) as empcount from Department as d
    join Employee as e on d.DepartmentID=e.DepartmentID
    group by d.Department_Name
    )
    select * from CTEtable
    where
    empcount>100;

CTE

 

 

In the above query, we didn’t mention the column name. If your inner query is given a distinct column name then there is no need to define the column name, otherwise you need to define it as shown below:

with CTEtable(deptname,empcount)
    as
    (
    select d.Department_Name as deptname, COUNT(e.empid) as empcount from Department as d
    join Employee as e on d.DepartmentID=e.DepartmentID
    group by d.Department_Name
    )
    select * from CTEtable
    where
    empcount>100;

 

 

In the above query, you specify 2 columns, so remember you need to specify the columns that select query is returning. If our inner select query is returning 3 columns then you need to specify these 3 columns in CTE.

 

CTE is only referenced by select, insert, update and delete statements which immediately follows the CTE expression.

 

In this with clause, you can create multiple CTE tables.  

with CTEtable(deptname,empcount)
    as
    (
    select d.deptname as deptname, COUNT(e.id) as empcount from tbldept as d
    join tblEmp as e on d.deptid=e.deptid
    group by d.deptname
    ),
    tblnew_hr(deptname,id)
    as
    (
    select d.deptname,e.id from tblEmp e join tbldept d on
    e.deptid=d.deptid
    )
    select * from CTEtable
    union
    select * from tblnew_hr

 

Updatable CTE

 

If your CTE is based on a single table then you can update using CTE, which in turn updates the underlying table. 

with update_cte
    as
    (
    select id, name, salary from tblEmp
    )
    update update_cte set salary=5555 where id =2
    select * from tblEmp;

 

If CTE is based on more than one table and the updates affect only the base table, then this is possible. 

with update_mul_cte
    as
    (
    select e.id,d.deptname,e.geneder from tblEmp e join tbldept d on e.deptid=d.deptid
    )
    update update_mul_cte set geneder='male' where id=2;
    select * from tblEmp;

 

But if you are going to update data in both tables which are present in CTE it will throw an error.  

with update_mul_cte
   as
   (
   select e.id,d.deptname,e.geneder from tblEmp e join tbldept d on e.deptid=d.deptid
   )
   update update_mul_cte set geneder='male',deptname='fff' where id=2;
   select * from tblEmp;

Update Multiple Table CTE Error 

 

Below is an error that is thrown by it.

 

Msg 4405, Level 16, State 1, Line 11

 

View or function ‘update_mul_cte’ is not updatable because the modification affects multiple base tables.