I think there is a syntax problem in your code. The colon in the first statement adds multiple following statements and hence updating all records in the first statement.
Remove the colon in the first line and comma between SET specifiers.
Try using the following statement after update:
UPDATE zemployee_jat SET prijs = zemployee_jat-prijs naam = zemployee_jat-naam WHERE employeeid = zemployee_jat-motorid.