When to use Common Table Expression (CTE)

Question

One example, if you need to reference/join the same data set multiple times you can do so by defining a CTE. Therefore, it can be a form of code re-use.

An example of self referencing is recursion: Recursive Queries Using CTE

For exciting Microsoft definitions Taken from Books Online:

A CTE can be used to:

• Create a recursive query. For more information, see Recursive Queries Using Common Table Expressions.

• Substitute for a view when the general use of a view is not required; that is, you do not have to store the definition in metadata.

• Enable grouping by a column that is derived from a scalar subselect, or a function that is either not deterministic or has external access.

• Reference the resulting table multiple times in the same statement.

I use them to break up complex queries, especially complex joins and sub-queries. I find I'm using them more and more as 'pseudo-views' to help me get my head around the intent of the query.

My only complaint about them is they cannot be re-used. For example, I may have a stored proc with two update statements that could use the same CTE. But the 'scope' of the CTE is the first query only.

Trouble is, 'simple examples' probably don't really need CTE's!

Still, very handy.

There are two reasons I see to use cte's.

To use a calculated value in the where clause. This seems a little cleaner to me than a derived table.

Suppose there are two tables - Questions and Answers joined together by Questions.ID = Answers.Question_Id (and quiz id)

WITH CTE AS
(
    Select Question_Text,
           (SELECT Count(*) FROM Answers A WHERE A.Question_ID = Q.ID) AS Number_Of_Answers
    FROM Questions Q
)
SELECT * FROM CTE
WHERE Number_Of_Answers > 0

Here's another example where I want to get a list of questions and answers. I want the Answers to be grouped with the questions in the results.

WITH cte AS
(
    SELECT [Quiz_ID] 
      ,[ID] AS Question_Id
      ,null AS Answer_Id
          ,[Question_Text]
          ,null AS Answer
          ,1 AS Is_Question
    FROM [Questions]

    UNION ALL

    SELECT Q.[Quiz_ID]
      ,[Question_ID]
      ,A.[ID] AS  Answer_Id
      ,Q.Question_Text
          ,[Answer]
          ,0 AS Is_Question
        FROM [Answers] A INNER JOIN [Questions] Q ON Q.Quiz_ID = A.Quiz_ID AND Q.Id = A.Question_Id
)
SELECT 
    Quiz_Id,
    Question_Id,
    Is_Question,
    (CASE WHEN Answer IS NULL THEN Question_Text ELSE Answer END) as Name
FROM cte    
GROUP BY Quiz_Id, Question_Id, Answer_id, Question_Text, Answer, Is_Question 
order by Quiz_Id, Question_Id, Is_Question Desc, Name

One of the scenarios I found useful to use CTE is when you want to get DISTINCT rows of data based on one or more columns but return all columns in the table. With a standard query you might first have to dump the distinct values into a temp table and then try to join them back to the original table to retrieve the rest of the columns or you might write an extremely complex partition query that can return the results in one run but in most likelihood, it will be unreadable and cause performance issue.

But by using CTE (as answered by Tim Schmelter on Select the first instance of a record)

WITH CTE AS(
    SELECT myTable.*
    , RN = ROW_NUMBER()OVER(PARTITION BY patientID ORDER BY ID)
    FROM myTable 
)
SELECT * FROM CTE
WHERE RN = 1

As you can see, this is much easier to read and maintain. And in comparison to other queries, is much better at performance.

Perhaps its more meaningful to think of a CTE as a substitute for a view used for a single query. But doesn't require the overhead, metadata, or persistence of a formal view. Very useful when you need to:

• Create a recursive query.
• Use the CTE's resultset more than once in your query.
• Promote clarity in your query by reducing large chunks of identical subqueries.
• Enable grouping by a column derived in the CTE's resultset

Here's a cut-and-paste example to play with:

WITH [cte_example] AS (
SELECT 1 AS [myNum], 'a num' as [label]
UNION ALL
SELECT [myNum]+1,[label]
FROM [cte_example]
WHERE [myNum] <=  10
)
SELECT * FROM [cte_example]
UNION
SELECT SUM([myNum]), 'sum_all' FROM [cte_example]
UNION
SELECT SUM([myNum]), 'sum_odd' FROM [cte_example] WHERE [myNum] % 2 = 1
UNION
SELECT SUM([myNum]), 'sum_even' FROM [cte_example] WHERE [myNum] % 2 = 0;

Enjoy

Today we are going to learn about Common table expression that is a new feature which was introduced in SQL server 2005 and available in later versions as well.

Common table Expression :- Common table expression can be defined as a temporary result set or in other words its a substitute of views in SQL Server. Common table expression is only valid in the batch of statement where it was defined and cannot be used in other sessions.

Syntax of declaring CTE(Common table expression) :-

with [Name of CTE]
as
(
Body of common table expression
)

Lets take an example :-

CREATE TABLE Employee([EID] [int] IDENTITY(10,5) NOT NULL,[Name] [varchar](50) NULL)

insert into Employee(Name) values('Neeraj')
insert into Employee(Name) values('dheeraj')
insert into Employee(Name) values('shayam')
insert into Employee(Name) values('vikas')
insert into Employee(Name) values('raj')

CREATE TABLE DEPT(EID INT,DEPTNAME VARCHAR(100))
insert into dept values(10,'IT')
insert into dept values(15,'Finance')
insert into dept values(20,'Admin')
insert into dept values(25,'HR')
insert into dept values(10,'Payroll')

I have created two tables employee and Dept and inserted 5 rows in each table. Now I would like to join these tables and create a temporary result set to use it further.

With CTE_Example(EID,Name,DeptName)
as
(
select Employee.EID,Name,DeptName from Employee 
inner join DEPT on Employee.EID =DEPT.EID
)
select * from CTE_Example

Lets take each line of the statement one by one and understand.

To define CTE we write "with" clause, then we give a name to the table expression, here I have given name as "CTE_Example"

Then we write "As" and enclose our code in two brackets (---), we can join multiple tables in the enclosed brackets.

In the last line, I have used "Select * from CTE_Example" , we are referring the Common table expression in the last line of code, So we can say that Its like a view, where we are defining and using the view in a single batch and CTE is not stored in the database as an permanent object. But it behaves like a view. we can perform delete and update statement on CTE and that will have direct impact on the referenced table those are being used in CTE. Lets take an example to understand this fact.

With CTE_Example(EID,DeptName)
as
(
select EID,DeptName from DEPT 
)
delete from CTE_Example where EID=10 and DeptName ='Payroll'

In the above statement we are deleting a row from CTE_Example and it will delete the data from the referenced table "DEPT" that is being used in the CTE.