>What is wrong with this? It replaces all the dates in covr with the same one, probably the first one in the patient table. I can do this in multiple lines with RELATION, but I need it to be one line.
>

>update covr set covr.ph_dob = patient.ptdob where ALLTRIM(covr.pat_no) = ALLTRIM(patient.ptpn)
>

I think you need something more like..

update covr set covr.ph_dob = (select patient.ptdob from patient where ALLTRIM(patient.ptpn) = ALLTRIM(covr.pat_no) )