It replaces with date in the current record in the 'patient' table because you did not specify any relations between 2 tables.

update covr set ph_dob = patient.ptdob ;
	FROM covr JOIN patient ON covr.pat_no == patient.ptpn

>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)
>