SQL Format Date User Defined Function Error - sql

Question

I have a user defined function that looks like this;
 
ALTER FUNCTION [dbo].[func_format_date]
(
    -- Add the parameters for the function here
    #input VARCHAR(6)
)
RETURNS VARCHAR(6)
AS
BEGIN
    -- Declare the return variable here
    DECLARE #mon CHAR(2),
     #day char(2),
        #year char(4),
        #output DATETIME
 
    -- Return the result of the function
    SELECT #mon = LEFT(#input, 2), #day =SUBSTRING(#input, 3,2), #year = RIGHT(#input,4)
 
SELECT #output = #year+#mon+#day
 
RETURN CONVERT(VARCHAR(6), #output, 12)
END
The goal is to be able to pass in a date say “022019” (mmddyy) and the function will format the date as such “190220” (yymmdd). This seems to work only sometime as if I pass in the following dates see the results below; it seems to be inconsistent in terms of the dates it accepts vs the dates that throw an error
022019 (Works fine)
032019 (Works fine)
021019 (Results in a “The conversion of a varchar data type to a datetime data type resulted in an out-of-range value.” error)
030519 (Results in a “The conversion of a varchar data type to a
datetime data type resulted in an out-of-range value.” error)
I have checked to ensure the default language is correct and it is. Can someone help me to figure this out?

Answer 1

First, the reason for this error - your #input parameter is varchar(6), you read 1st and 2nd characters as month, 3rd and 4th as day, and 3rd, 4th, 5th and 6th as year. When #input is equal to '021019', your year will be 1019 and this is outside the datetime datatype range.
Second, change your logic:
make your #input parameter varchar(8)
generate datetime value using DATETIMEFROMPARTS() function if you have four digit year
If you want to fix your function, next may help:
ALTER FUNCTION [dbo].[func_format_date]
(
#input VARCHAR(6)
)
RETURNS VARCHAR(6)
AS
BEGIN
-- Declare the return variable here
DECLARE
#mon VARCHAR(2),
#day VARCHAR(2),
#year VARCHAR(2),
#output DATETIME
-- Return the result of the function
SELECT
#mon = SUBSTRING(#input, 1, 2),
#day = SUBSTRING(#input, 3, 2),
#year = SUBSTRING(#input, 5, 2)
-- Generate datetime value
SELECT #output = CONVERT(datetime, #year + #mon + #day, 12)
-- or if your dates are after 2000
--SELECT #output = DATETIMEFROMPARTS(2000 + #year, #mon, #day, 0, 0, 0, 0)
-- Return value
RETURN CONVERT(VARCHAR(6), #output, 12)
END

Answer 2

You Can Simply your function using below Stmt.
If no Date Validation Required you can interchange values.
select RIGHT(#input,2) + LEFT(#input, 2) + SUBSTRING(#input, 3,2)
If Date Validation Required use below.
select Convert(varchar, Convert(datetime, LEFT(#input, 2) + '/' + SUBSTRING(#input, 3,2) + '/' + RIGHT(#input,2), 1), 12)