How to update a column in a table A using the value from another table B wherein the relationship between tables A & B is 1:N by using max() function - sql

Question

I have two tables namely loan_details and loan_his_mapping with 1:N relationship. I need to set the hhf_request_id of loan_details table by the value which is present in the loan_his_mapping table for each loan.
Since the relationship is 1:N , I want to consider the record for each loan from loan_his_mapping table with two conditions mentioned below. The table definitions are as follows:
CREATE TABLE public.loan_details
(
loan_number bigint NOT NULL,
hhf_lob integer,
hhf_request_id integer,
status character varying(100),
CONSTRAINT loan_details_pkey PRIMARY KEY (loan_number)
);
CREATE TABLE public.loan_his_mapping
(
loan_number bigint NOT NULL,
spoc_id integer NOT NULL,
assigned_datetime timestamp without time zone,
loan_spoc_map_id bigint NOT NULL,
line_of_business_id integer,
request_id bigint,
CONSTRAINT loan_spoc_his_map_id PRIMARY KEY (loan_spoc_map_id),
CONSTRAINT fk_loan_spoc_loan_number_his FOREIGN KEY (loan_number)
REFERENCES public.loan_details (loan_number) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION );
The joining conditions while updating are:
The Records of loan_details with hhf_lob = 4 and status='Release'
I should consider that record for updating value among 'N' number of records from loan_his_mapping table with value max(loan_spoc_map_id) for each loan.
The query I have right now
update lsa_loan_details ldet
set hhf_request_id = history.request_id
from loan_his_mapping history
where ldet.loan_number = history.loan_number and ldet.status='Release' and ldet.hhf_lob=4 and
history.line_of_business_id=4 ;
I want to know how to use that record for each loan from loan_his_mapping with max(loan_spoc_map_id) to update column of loan_details table. Please Assist!

Answer 1

You need a sub-query to fetch the row corresponding to the highest loan_spoc_map_id
Something along the lines:
update loan_details ldet
set hhf_request_id = history.request_id
from (
select distinct on (loan_spoc_map_id) loan_number, request_id
from loan_his_mapping lhm
where lhm.line_of_business_id = 4
order by loan_spoc_map_id desc
) as history
where ldet.loan_number = history.loan_number
and ldet.status = 'Release'
and ldet.hhf_lob = 4;