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User Defined Functions
Written by Doug Carpenter on 12 October 2000
This article covers all the basics of User Defined Functions. It discusses how (and why) to create them and when to use them. It talks about scalar, inline table-valued and multi-statement table-valued functions. (This article has been updated through SQL Server 2005.)
With SQL Server 2000, Microsoft
has introduced the concept of User-Defined Functions that
allow you to define your own T-SQL functions that can accept
zero or more parameters and return a single scalar data value
or a table data type.
What Kind of User-Defined Functions can I Create?
There are three types of User-Defined functions in SQL Server
2000 and they are Scalar, Inline Table-Valued and Multi-statement
How do I create and use a Scalar User-Defined
A Scalar user-defined function returns one of the scalar
data types. Text, ntext, image and timestamp data types
are not supported. These are the type of user-defined functions
that most developers are used to in other programming languages.
You pass in 0 to many parameters and you get a return value.
Below is an example that is based in the data found in the
NorthWind Customers Table.
CREATE FUNCTION whichContinent
declare @Return varchar(30)
select @return = case @Country
when 'Argentina' then 'South America'
when 'Belgium' then 'Europe'
when 'Brazil' then 'South America'
when 'Canada' then 'North America'
when 'Denmark' then 'Europe'
when 'Finland' then 'Europe'
when 'France' then 'Europe'
Because this function returns a scalar value of a varchar(30)
this function could be used anywhere a varchar(30) expression
is allowed such as a computed column in a table, view, a
T-SQL select list item. Below are some of the examples that
I was able to use after creating the above function definition.
Note that I had to reference the dbo in the function name.
select dbo.WhichContinent(Customers.Country), customers.*
create table test
Continent as (dbo.WhichContinent(Country)))
insert into test (country)
select * from test
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
USA North America
Stored procedures have long given us the ability to pass
parameters and get a value back, but the ability to use
it in such a variety of different places where you cannot
use a stored procedure make this a very powerful database
object. Also notice the logic of my function is not exactly
brain surgery. But it does encapsulate the business rules
for the different continents in one location in my application.
If you were to build this logic into T-SQL statements scattered
throughout your application and you suddenly noticed that
you forgot a country (like I missed Austria!) you would
have to make the change in every T-SQL statement where you
had used that logic. Now, with the SQL Server User-Defined
Function, you can quickly maintain this logic in just one
How do I create and use an Inline Table-Value
An Inline Table-Value user-defined function returns a table
data type and is an exceptional alternative to a view as
the user-defined function can pass parameters into a T-SQL
select command and in essence provide us with a parameterized,
non-updateable view of the underlying tables.
CREATE FUNCTION CustomersByContinent
SELECT dbo.WhichContinent(Customers.Country) as continent,
WHERE dbo.WhichContinent(Customers.Country) = @Continent
SELECT * from CustomersbyContinent('North America')
SELECT * from CustomersByContinent('South America')
SELECT * from customersbyContinent('Unknown')
Note that the example uses another function (WhichContinent)
to select out the customers specified by the parameter of
this function. After creating the user-defined function,
I can use it in the FROM clause of a T-SQL command unlike
the behavior found when using a stored procedure which can
also return record sets. Also note that I do not have to
reference the dbo in my reference to this function. However,
when using SQL Server built-in functions that return a table,
you must now add the prefix :: to the name of the function.
Example from Books Online:
Select * from ::fn_helpcollations()
How do I create and use a Multi-statement
Table-Value User-Defined Function?
A Multi-Statement Table-Value user-defined function returns
a table and is also an exceptional alternative to a view
as the function can support multiple T-SQL statements to
build the final result where the view is limited to a single
SELECT statement. Also, the ability to pass parameters into
a T-SQL select command or a group of them gives us the capability
to in essence create a parameterized, non-updateable view
of the data in the underlying tables. Within the create
function command you must define the table structure that
is being returned. After creating this type of user-defined
function, I can use it in the FROM clause of a T-SQL command
unlike the behavior found when using a stored procedure
which can also return record sets.
CREATE FUNCTION dbo.customersbycountry ( @Country varchar(15) )
@CustomersbyCountryTab table (
[CustomerID] [nchar] (5), [CompanyName] [nvarchar] (40),
[ContactName] [nvarchar] (30), [ContactTitle] [nvarchar] (30),
[Address] [nvarchar] (60), [City] [nvarchar] (15),
[PostalCode] [nvarchar] (10), [Country] [nvarchar] (15),
[Phone] [nvarchar] (24), [Fax] [nvarchar] (24)
INSERT INTO @CustomersByCountryTab
WHERE country = @Country
DECLARE @cnt INT
SELECT @cnt = COUNT(*) FROM @customersbyCountryTab
IF @cnt = 0
INSERT INTO @CustomersByCountryTab (
VALUES ('','No Companies Found','','','','','','','','')
SELECT * FROM dbo.customersbycountry('USA')
SELECT * FROM dbo.customersbycountry('CANADA')
SELECT * FROM dbo.customersbycountry('ADF')
What are the benefits of User-Defined
The benefits to SQL Server User-Defined functions are numerous.
First, we can use these functions in so many different places
when compared to the SQL Server stored procedure. The ability
for a function to act like a table (for Inline table and
Multi-statement table functions) gives developers the ability
to break out complex logic into shorter and shorter code
blocks. This will generally give the additional benefit
of making the code less complex and easier to write and
maintain. In the case of a Scalar User-Defined Function,
the ability to use this function anywhere you can use a
scalar of the same data type is also a very powerful thing.
Combining these advantages with the ability to pass parameters
into these database objects makes the SQL Server User-Defined
function a very powerful tool.
So, if you have ever wanted to use the results of a stored
procedure as part of a T-SQL command, use parameterized
non-updateable views, or encapsulate complex logic into
a single database object, the SQL Server 2000 User-Defined
function is a new database object that you should examine
to see if its right for your particular environment.