The basic syntax of CTE - Pinal Dave

Ahmedabad SQL Server User Group
Ahmedabad, Gujarat, India
September 20, 2008
Pinal Dave
Microsoft MVP –SQL Server
Founder – SQLAuthority.com
pinal@SQLAuthority.com
© Pinal Dave
September 20, 2008

Speaker Profile
• Microsoft Most Valuable Professional –SQL Server
• Masters of Science (Computer Networks) –
University of Southern California, Los Angeles, USA.
• Bachelors of Engineering (Electronics &
Communication) –Nirma Institutes of Technology,
Ahmedabad, India.
• MCP, MCDBA, MCAD
• Founder – SQLAuthority.com
• 5+ Years of experience as Database Administrator
© Pinal Dave
September 20, 2008

Agenda
• Introduction
• Speaker Profile
• Agenda
• SQL Server Common Table Expression(CTE)
• Recursive CTE
• Summary
• Questions & Answers
• References
• End Note
© Pinal Dave
September 20, 2008

Common Table Expression (CTE)
• A Common Table Expression (CTE) is an expression
that can be thought of as a temporary result set
which is defined within the execution of a single
SQL statement. A CTE is similar to a derived table in
that it is not stored as an object and lasts only for
the duration of the query.
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September 20, 2008

CTE Advantages
‐Using CTE improves the readability and makes
maintenance of complex queries easy.
‐The query can be divided into separate, simple, logical
building blocks which can be then used to build more
complex CTEs until final result set is generated.
‐CTE can be defined in functions, stored procedures,
triggers or even views.
‐After a CTE is defined, it can be used as a Table or a View
and can SELECT, INSERT, UPDATE or DELETE Data.
© Pinal Dave
September 20, 2008

Basic Structure of CTE
The basic syntax of CTE:
;WITH expression_name [( column_name [,...n] )]
AS
(CTE_query_definition)
The statement to use the CTE is:
SELECT
FROM expression_name;
© Pinal Dave
September 20, 2008

CTE Example
USE AdventureWorks
GO
;WITH EmployeeDepartment_CTE AS (
SELECT EmployeeID,DepartmentID,ShiftID
FROM HumanResources.EmployeeDepartmentHistory
WHERE EmployeeID = 127
)
SELECT ecte.EmployeeId,ed.DepartmentID, ed.Name,ecte.ShiftID
FROM HumanResources.Department ed
INNER JOIN EmployeeDepartment_CTE ecte ON ecte.DepartmentID =
ed.DepartmentID
GO
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September 20, 2008

CTE Example Resultset
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September 20, 2008

Recursive CTE
• Process in which query executes itself is referred as
Recursive Process in which the result set is based on
the output of the base query.
• Unlike a derived table, a CTE can be selfreferencing
and can be referenced multiple times in
the same query which means in Recursive CTE, the
result of CTE is repeatedly used to get the final
result set.
© Pinal Dave
September 20, 2008

Recursive CTE Example
;WITH Emp_CTE AS (
SELECT EmployeeID, ContactID, LoginID, ManagerID, Title, BirthDate
FROM HumanResources.Employee
WHERE ManagerID IS NULL
UNION ALL
SELECT e.EmployeeID, e.ContactID, e.LoginID, e.ManagerID, e.Title,e.BirthDate
FROM HumanResources.Employee e
INNER JOIN Emp_CTE ecte ON ecte.EmployeeID = e.ManagerID
)
SELECT *
FROM Emp_CTE
GO
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September 20, 2008

Recursive CTE Example Resultset
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September 20, 2008

Recursive CTE Example Explanation
• In the given example Emp_CTE is a Common Expression Table, the base
record for the CTE is derived by the first sql query before UNION ALL.
The result of the query gives you the EmployeeID which don’t have
ManagerID.
• Second query after UNION ALL is executed repeatedly to get results and
it will continue until it returns no rows. For above e.g. Result will have
EmployeeIDs which have ManagerID (ie, EmployeeID of the first
result). This is obtained by joining CTE result with Employee table on
columns EmployeeID of CTE with ManagerID of table Employee.
• This process is recursive and will continue till there is no ManagerID who
doesn’t have EmployeeID.
© Pinal Dave
September 20, 2008

Different ways of using CTE 1
;WITH First_CTE
SELECT Statement,
Second_CTE
SELECT Statement,
Third_CTE
SELECT Statement,
SELECT Statement
FROM First_CTE
INNER JOIN Second_CTE
INNER JOIN Third_CTE
© Pinal Dave
September 20, 2008

Different ways of using CTE 2
;WITH First_CTE
SELECT Statement,
Second_CTE
SELECT Statement
FROM First_CTE,
Third_CTE
SELECT Statement
FROM Second_CTE,
SELECT Statement
FROM Third_CTE
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September 20, 2008

Your Ideas on CTE
Your Suggestions
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September 20, 2008

Summary
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September 20, 2008

Questions and Answers
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September 20, 2008

References
• Microsoft Book Online
• SQLAuthority.com
• SQL Server 2008
© Pinal Dave
September 20, 2008

End Note
• Thank You!
• For any questions or queries contact me at
pinal@SQLAuthority.com
© Pinal Dave
September 20, 2008