T-SQL replace NULL values in result
One way to accomplish this is to use LEFT OUTER JOIN along with COALESCE. To get the missing values in the J_ID and K_ID column you can join to R_ID and JK_ID. To get the missing values in the Y_ID column you can join to JK_ID and R_ID.

Something close to the following should work:

SELECT
  cus.USER_ID
, cus.NAME
, COALESCE(cus.J_ID, r_to_jk.J_ID) AS J_ID
, COALESCE(cus.K_ID, r_to_jk.K_ID) AS K_ID
, COALESCE(cus.Y_ID, r_to_y.Y_ID) AS J_ID
FROM Customers cus
LEFT OUTER JOIN R_ID y_to_r ON cus.Y_ID = y_to_r.Y_ID
LEFT OUTER JOIN JK_ID r_to_jk ON y_to_r.R_ID = r_to_jk.R_ID
LEFT OUTER JOIN JK_ID jk_to_r ON cus.J_ID = jk_to_r.J_ID AND cus.K_ID = jk_to_r.K_ID
LEFT OUTER JOIN R_ID r_to_y ON jk_to_r.R_ID = r_to_y.R_ID;