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SQL 2000 User Defined Function Primer



Many programming languages have supported User Defined Functions for years, but they

are new to SQL Server 2000. In this article we will look at some of the

ways functions can be used within SQL Server 2000.

This document was developed against SQL Server 2000 Beta 2, so some points may be subject to change in the release version.

What are functions?

Functions are pre-prepared pieces of code that may accept parameters, and

always return a value. Older versions of SQL Server supply some built-in

functions, such as getdate() and object_name() but to be able to

define your own functions is a new feature in SQL Server 2000. 

Throughout the text I will use the

abbreviation UDF for User Defined Function.

All functions (not just UDFs) are split into two groups, deterministic and non-deterministic.

Deterministic functions will always return the save value if provided with the

same parameters. Non-deterministic functions may produce different results each

time they are called, even if any supplied parameters are unchanged.

SQL Server Books Online also uses the term "scalar functions" -

these are simply functions that return one value, as opposed to functions that

return a table.

getdate() is a good example of both a scalar and a non-deterministic function - call it

twice in a row and you will get two different date-time results, but only one

value from each call.

A simple UDF

It's not uncommon to want to know the number of days in particular month, so

let's look at a simple function to do this. Here is the code:

create function DaysInMonth (@when datetime) 

returns int 



declare@rv int

if datepart(month, @when) = 2 


    if datepart(year, @when) % 400 = 0 

    select @rv = 29 


    if datepart(year, @when) % 100 = 0 

    select @rv = 28 


    if datepart(year, @when) % 4 = 0 

    select @rv = 29 


    select @rv = 28 

end -- if



select @rv = case (datepart(month, @when)) 

    when 4 then 30 

    when 6 then 30 

    when 9 then 30 

    when 11 then 30 

    else 31 

    end -- case 

end -- else

return @rv

END -- fn def 

To execute the function, 

select dbo.DaysInMonth('1 may 2000') 

select dbo.DaysInMonth(getdate())

As you can see, most of the code could have come from any SQL Server stored

procedure. The new features are:

    create function DaysInMonth (@when


    returns int 

Clearly this is a function definition header - the returns int states

that the function returns a single integer value.

    return @rv

The return @rv command must be the final line of the function, and

it's this instruction that sends the output value back to the call. Almost any

SQL Server data type can be returned by a UDF.

Using function calls

You can code a function call anywhere you could code a variable or literal of

the same data type as the function returns. For example, to calculate the number

of days between now and the end of the month:

    select dbo.DaysInMonth(getdate()) - datepart(day,


Note the repeated use of the owner qualification before the UDF name, (dbo.DaysInMonth()

instead of DaysInMonth() - This qualification is a SQL Server requirement

in Beta 2, and will probably go forward into the release version.

Functions within functions

You can nest a function within another of you so wish.

create function daysTillMonthEnd (@when Datetime)

returns int



return dbo.DaysInMonth(@when) - datepart(day, @when)


Restrictions on using functions within functions

 There is a restriction on this which I ran straight into when I first started to play with UDFs. You cannot use a built-in non-deterministic

function within a user defined function. When I made my first stab at coding

the DaysInMonth function shown above, I tried to use the getdate()

function as a default in case no parameters were supplied in the function call.

SQL Server will not allow this.  

Using other data types

You can return almost any data type in a function, including User Defined Data

Types. The following simple example returns a UDDT based on the varchar data


sp_addtype varstring, 'varchar(32)'


create function capitalise (@string varstring) 

returns varstring



    return UPPER(left(@string, 1)) + 

           LOWER(substring (@string, 2, datalength(@string)))



select dbo.capitalise ('abcdef')

SQL Server 2000 will not allow you to return text, image, cursor or timestamp

data types from a function.

Recursive functions

Functions have the ability to call themselves, or "recurse". The

example below works out the factorial of the integer input. 

create function factorial (@i bigint) 

returns bigint



    declare @internalI bigint

    if @i > 20 OR @i IS NULL 

        select @internalI = NULL


    if @i < 2 

        select @internalI = @i


        select @internalI = @i * dbo.factorial(@i - 1)

    return @internalI

END -- fn def


select dbo.factorial (3)

select dbo.factorial (8)

For the non-mathematically inclined, factorials are what you get when you

multiply together all the whole numbers between 1 and the number you thought of.

For example:

  • The Factorial of 3 = 1 x 2 x 3 = 6
  • The Factorial of 8 = 1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 = 40320
  • Functions can recurse up to level 32 deep, after which SQL Server will generate an

    error. The example above is restricted to 20 times because Factorial

    21 is too big for the bigint data type.

    Passing tables out of functions

    Create a function that passes back a table and what you end up with is a kind

    of "parameterized view" - something you can treat as a view, but which

    will return different values depending on the parameters passed in. The

    following example for the PUBS database takes an author_id as it's parameter and

    returns a table containing all the author IDs that have collaborated with them,

    and the title_id they worked together on.

    create function collaborator (@au_id char(11))

    returns @collaborator table (







        insert     @collaborator

        select     l.au_id, l.title_id, r.au_id

        from       titleauthor l inner join titleauthor r

        on         l.title_id = r.title_idAND l.au_id <> r.au_id

        where      l.au_id = COALESCE(@au_id, l.au_id)

        orderby   l.title_id, l.au_id


    END -- fn def

    select * from dbo.collaborator('472-27-2349')

    select * from dbo.collaborator(NULL)

    Note the use of the COALESCE function. We use this to return data for all

    authors that have collaborated on a book if no specific author_id is given.

    You could code the line create function collaborator (@au_id char(11) =

    NULL) to define a default value for the parameter, and SQL Server would

    accept this. However, if you then try calling the function without parameters (a

    perfectly valid thing to do with stored procedure parameters) then SQL Server

    will complain. Instead you have to code dbo.collaborator(DEFAULT) for the

    function call.

    In-line table functions

    SQL Server differentiates between "user defined functions that return a

    table" and "in-line table functions". The previous example is a

    "user defined functions that return a table". We can re-code this as an

    in-line function as follows:

    create function collaborator_inline (@au_id char(11))

    returns table as

    RETURN (

        select  l.au_id as "author", 

                l.title_id as "title", 

                r.au_id as "co-author"

        from    titleauthor l inner join titleauthor r

        on      l.title_id = r.title_id AND l.au_id <> r.au_id

        where   l.au_id = COALESCE(@au_id, l.au_id)

    --  order by l.title_id, l.au_id



    select * from dbo.collaborator_inline('724-80-9391')

    select * from dbo.collaborator_inline(NULL)

    This syntax is more compact than the previous example, but also more

    restrictive. While the "user defined functions that return a table"

    can include any amount of code, the in-line version is restricted to a single

    select statement. Even the "order by" clause needs to be removed from

    the in-line version.

    Table function in joins

    Parameterized functions that return tables may be part of a standard SQL join


    select * 

    from dbo.collaborator('472-27-2349') inner join authors as a 

    on partner_author = a.au_id

    Functions as columns in views

    UDFs can be used as columns in a view. 

    Building on the daysInMonth() function we defined earlier, and

    a new function startOfMonth() function, we can create this

    NORTHWIND database view showing daily average sales figures per month without

    going to excessive lengths to cater for months with different numbers of days.

    create function startOfMonth (@when datetime)

    returns smalldatetime as 


        return dateadd (day, 1 + datepart(day, @when) * -1, @when)

    END -- fn def


    create view monthlyAverages as


        sum(quantity) / 


        as dailyAvgSale

        from (selectdbo.startofMonth (orderDate) 

            as month,



            fromorders o 

            inner join "order details" od

            on o.OrderID = od.OrderID

        ) as derived

    group by month


    select * from monthlyAverages order by month

    Functions and constraints

    You can use functions as to define constraints, provided that both the table

    and the function share the same owner. This example defines a function called

    " midnight" that truncates a date-time to midnight, then uses this in

    a table's check constraint to ensure that the prevShutdown column in test_table

    is always at least a day earlier then the value in the startup column.

    create function midnight (@when smalldatetime)

    returns smalldatetime



        return convert(varchar(11), @when)

    END -- fn def


    create table test_table (

       id               int identity,

       prevShutdown     smalldatetime,



       CONSTRAINT       chk_midnight CHECK (prevShutdown < dbo.midnight(startup))



    You can also use a function as the basis for a default constraint, but

    presently you are limited to using constants in the function parameters, which

    restricts their usefulness.

    Schema bound functions

    Particularly with older versions of SQL Server, you could get into trouble if

    you defined one object  that was dependent on another, then dropped or

    changed the subordinate object. (In plain English: if you created a view on a

    table, then dropped the table, the view would still exist but it would obviously

    not work)

    Schema binding tries to put a stop to this. If you create a function and

    specify it is Schema Bound, then the objects it depends on cannot be altered

    without you first dropping the schema binding. There are limitations as to when

    this will work (it will not work across multiple databases for example) but it

    is still a very useful feature.

    Schema binding is invoked at function creation time. We can change our collaborator()

    function to a schema bound version as shown below. Note that in addition to

    the with schemabinding

    option, we need to specify a two part name (dbo.titleauthor

    instead of just titleauthor) for all referenced objects.

    create function collaborator_sb (@au_id char(11))

    returns table 

    with schemabinding


    RETURN (

        select  l.au_id as "author", 

                l.title_id as "title", 

                r.au_id as "co-author"

        from    dbo.titleauthor l inner join

    dbo.titleauthor r

        on      l.title_id = r.title_id AND l.au_id <> r.au_id

        where   l.au_id = COALESCE(@au_id, l.au_id)

    -- order by l.title_id, l.au_id


    As long as schema binding is in place for this function, any attempt to

    change the structure of the titleauthor table will generate an error.

    Overloading functions

    Overloading is the practice of having multiple functions with the name name,

    each doing different things. The '+' operator is overloaded as it can both add

    up numbers and concatenate strings.

    select 1 + 2 + 3

    select '1' + '2' + '3'

    In Beta 2 at least, overloading functions is a non-starter due to the UNIQUE

    constraint on the sysobjects columns name and uid. There is

    some discussion in the Beta support newsgroups about implementing this in

    future, but I would not hold my breath.

    About the author

    Neil Boyle is an independent SQL Server consultant working out of London,

    England. Neil's free SQL Server guide is available on-line at http://www.impetus-sql.co.uk


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