Most people that use SQL Server are familiar with formatting dates inside SQL Server. The common approach is:

convert(varchar, getdate(), 106) -- Displays a date in the format 25 Jan 2008

That is all fine and good if your requirements are satisfied with the formats available inside SQL Server. SQL Server does provide a decent number of formats. Here is the list right from the SQL Server help:

Wouldn't you know it, my requirements were not :-) I tried working with the built in formats and had to roll my own after all anyway. So here is the function I came up with: Please note: The function below relies on using regular expressions inside SQL Server. SQL Server does not have built in regular expression support so the function below relies on the method described in my other blog post about Using Regular Expression in SQL Server.

/*
Type:Function
Name:dbo.fnFormatDate
Author:Boyan Kostadinov
Created:01.25.2008
Dependencies:master.dbo.fn_pcre_replace
Parameters:@inputDate(datetime)
- The date to format

@formatString(varchar)
- the format string to use (Examples "dd mm yyyy", "mmm.dd.yy")
Description:

Formats a given date based on the format specified in @formatString
d - one digit day (when applicable)
dd- two digit day
ddd- short day name
dddd- long day name
m- one digit month (when applicable)
mm- two digit month
mmm- short month name
mmmm- long month name
yy- two digit year
yyyy- four digit year
*/
create function dbo.fnFormatDate
(
@inputDate datetime,
@formatString varchar(25)
)
returns varchar(20) as
begin
declare @returnValue varchar(25)

-- Declare local vairables
declare @formattedDate varchar(25),
@day varchar(20), @month varchar(20), @year varchar(20),
@dayFormat varchar(5), @monthFormat varchar(5), @yearFormat varchar(5)

set @dayFormat = ''
set @monthFormat = ''
set @yearFormat = ''

-- Convert the supplied date to day mon year (25 Jan 2008)
set @formattedDate = convert(varchar, @inputDate, 106)

-- If the format string contains a format for the day
if charindex('d', @formatString) > 0
-- Get the day format string
set @dayFormat = master.dbo.fn_pcre_replace(@formatString, '.*?(d{1,4}).*', '$1')

-- If the format string contains a format for the month
if charindex('m', @formatString) > 0
-- Get the month format string
set @monthFormat = master.dbo.fn_pcre_replace(@formatString, '.*?(m{1,4}|M{1,4}).*', '$1')

-- If the format string contains a format for the year
if charindex('y', @formatString) > 0
-- Get the year format string
set @yearFormat = master.dbo.fn_pcre_replace(@formatString, '.*?(y{2,4}).*', '$1')

-- Format the day value based on the format string for the day
select@day =
case @dayFormat
when 'dd' then master.dbo.fn_pcre_replace(@formattedDate, '^(\d+).*', '$1')
when 'ddd' then substring(datename(dw, @formattedDate), 1, 3)
when 'dddd' then datename(dw, @formattedDate)
else convert(varchar, day(@formattedDate))
end

-- Format the month value based on the format string for the month
select@month =
case @monthFormat
when 'mm' then master.dbo.fn_pcre_replace(convert(varchar, @inputDate, 101), '^(\d+)/.*', '$1')
when 'mmm' then master.dbo.fn_pcre_replace(@formattedDate, '\d+\s(\w+)\s\d+', '$1')
when 'mmmm' then datename(m, @formattedDate)
else convert(varchar, month(@formattedDate))
end

-- Format the year value based on the format string for the year
select@year =
case @yearFormat
when 'yy' then substring(convert(varchar, year(@formattedDate)), 3, 2)
else convert(varchar, year(@formattedDate))
end

set @returnValue = @formatString

-- If the day format was specified
if @dayFormat <> ''
-- Replace the day format string with the actual day value
set @returnValue = master.dbo.fn_pcre_replace(@returnValue, @dayFormat, @day)

-- If the month format was specified
if @monthFormat <> ''
-- Replace the month format string with the actual month
set @returnValue = master.dbo.fn_pcre_replace(@returnValue, @monthFormat, @month)

-- If the year format was specified
if @yearFormat <> ''
-- Replace the year format string with the actual year
set @returnValue = master.dbo.fn_pcre_replace(@returnValue, @yearFormat, @year)

-- Return the formated value
return @returnValue
end

To test this function, I created a table that hold the following date format string:

formatString             -------------------------dd MMM yydd MMM yyyydd-MM-yydd-MM-yyyydd.MM.yydd.MM.yyyydd/MM/yydd/MM/yyyyddMMMyyddMMMyyyyMM-dd-yyMM-dd-yyyyMM/dd/yyMM/dd/yyyyMMM dd yyyyMMM dd, yyMMM dd, yyyyMMMdd,yyyyMMMddyyyyyy.MM.ddyy/MM/ddyyMMddyyyy-MM-ddyyyy.MM.ddyyyy/MM/ddyyyyMMddMMMyyyy

I tested the function with the simple SQL query:

select df.formatString, dbo.fnFormatDate(getdate(), df.formatString) as formattedDatefrom dateFormats as df

And here are the results:

formatString              formattedDate       ------------------------- --------------------dd MMM yy                 25 Jan 08dd MMM yyyy               25 Jan 2008dd-MM-yy                  25-01-08dd-MM-yyyy                25-01-2008dd.MM.yy                  25.01.08dd.MM.yyyy                25.01.2008dd/MM/yy                  25/01/08dd/MM/yyyy                25/01/2008ddMMMyy                   25Jan08ddMMMyyyy                 25Jan2008MM-dd-yy                  01-25-08MM-dd-yyyy                01-25-2008MM/dd/yy                  01/25/08MM/dd/yyyy                01/25/2008MMM dd yyyy               Jan 25 2008MMM dd, yy                Jan 25, 08MMM dd, yyyy              Jan 25, 2008MMMdd,yyyy                Jan25,2008MMMddyyyy                 Jan252008yy.MM.dd                  08.01.25yy/MM/dd                  08/01/25yyMMdd                    080125yyyy-MM-dd                2008-01-25yyyy.MM.dd                2008.01.25yyyy/MM/dd                2008/01/25yyyyMMdd                  20080125MMMyyyy                   Jan2008