Question

I need an expression which will convert a text (VARCHAR) column to a DATETIME if, and only if, it matches dd/MM/yyyy, d/MM/yyyy, dd/M/yyyy or d/M/yyyy. If it doesn’t match then I want a NULL.

I have this…

CASE ISDATE([DateField]) 
  WHEN 1 THEN CONVERT(DATETIME,[DateField],103) 
  ELSE NULL 
END

However this fails for '15/04/76' for example – with a “Conversion failed when converting datetime from character string” error – whereas I would want it to return NULL

Example output

'1/6/1976'     -> 1976-06-01
'01/06/1976'   -> 1976-06-01
'13/06/2001'   -> 2001-06-13
'06/13/2001'   -> NULL
'13/06/76'     -> NULL

Is there a way of forcing ISDATE to validate a given format?

The documentation seems to suggest so…

ISDATE is deterministic only if used with the CONVERT function, the
CONVERT style parameter is specified and style is not equal to 0, 100,
9, or 109.

But ISDATE only takes one argument, so how do I “use it with CONVERT function” if I am not doing so already?

Answer 1

You could do a nested case statement here. The first could check to see if you have a 10 character string 2 for day, 2 for month, 4 for year and 2 for separators = 10 characters.

SET DATEFORMAT DMY;

    Case When DateField Like '%/%/[0-9][0-9][0-9][0-9]' 
         Then Case When IsDate(DateField) = 1 
                   Then CONVERT(DATETIME,[DateField],103) 
                   End
         End

Revised: I changed the code to use a like search which forces there to be a /YYYY at the end of the string, and then does an IsDate check to allow for a single day and/or month.