text: UPDATE drug_concept_stage
  SET valid_start_date = TO_DATE('19700101', 'yyyymmdd');
  
  UPDATE drug_concept_stage dcs
  SET valid_start_date = TO_DATE(d.bdzul, 'dd.mm.yyyy')
  FROM (
  	SELECT enr,
  		bdzul
  	FROM source_table
  	) d
  WHERE d.enr = dcs.concept_code;