UPDATE T
SET T.field_1 = CASE WHEN .. THEN t2.field_1 ELSE t3.field_1 END 
FROM table_1 AS T
  INNER JOIN table_2 AS t2
	 ON t.F_t2_id  = t2.id
  INNER JOIN table_3 AS t3
     ON t.F_t3_id  = t3.id

 UPDATE table_1 AS T
SET T.field_1 = CASE WHEN .. THEN t2.field_1 ELSE t3.field_1 END 
FROM table_2 AS t2
	 table_3 AS t3, 
 WHERE t.F_t2_id  = t2.id
  AND t.F_t3_id  = t3.id   

UPDATE T
SET T.field_1 = CASE WHEN .. THEN t2.field_1 WHERE .. THEN ISNULL(t4.field_1,0) ELSE t3.field_1 END 
FROM table_1 AS T
  INNER JOIN table_2 AS t2
	 ON t.F_t2_id  = t2.id
  INNER JOIN table_3 AS t3
     ON t.F_t3_id  = t3.id	 
   LEFT JOIN table_4 AS t4
     ON  t.F_t4_id  = t4.id