Fill the blank from historical data

[peac3]

Hi,

I have the data needs to be backed fill based on historical data because the data is not consistent (some day it's populated and some day it's not)
Below I have two tables (Transaction table and Account_Location table):

Transaction - capture daily transaction with account and location information.
TRAN_D         ACCT_N      LOCN_N      TRAN_A
2018-02-08     1234        A123        125
2018-02-07                 B123        125 
2018-02-06                 C123        100 
2018-02-05     3456        D123        500
2018-02-04                 B123        600
2018-02-03                 C123        100
2018-02-02     5678        E123        100
2018-02-01                 F123        200 

Account_Location_Hist - capture historical account and location, The account and location are unique for each date
LOCN_D         ACCT_N      LOCN_N      
2018-02-08     1234        A123
2018-02-07     9999        B123        
2018-02-07     1234        A123
2018-02-06     8888        B123
2018-02-05     7777        C123
2018-02-03     6666        B123
2018-02-03     1234        A123
2018-02-02     4444        C123        
2018-02-02     1234        A123
2018-02-01     1234        A123
2018-01-30     3333        F123        
2018-01-29     2222        F123

We would like to backfill the transaction records that have blank account number with location information and join the account location history.
But we would like to pick up the most recent previous day account number (which is max(locn_d-1)) from historical data in account location history.
Then display how many days different from transaction date.

so the expected output after back fill would be

Transaction - capture daily transaction with account and location information.
TRAN_D         ACCT_N      LOCN_N      TRAN_A    DATE_DIFF
2018-02-08     1234        A123        125
2018-02-07     [COLOR=#EF2929]8888[/color]        B123        125           1  -- (picked up from 2018-02-06)
2018-02-06     [COLOR=#EF2929]7777[/color]        C123        100           1  -- (picked up from 2018-02-05)
2018-02-05     3456        D123        500
2018-02-04     [COLOR=#EF2929]6666[/color]        B123        600           1  -- (picked up from 2018-02-03)
2018-02-03     [COLOR=#EF2929]4444[/color]        C123        100           1  -- (picked up from 2018-02-02)
2018-02-02     5678        E123        100          
2018-02-01     [COLOR=#EF2929]3333[/color]        F123        200           2  -- (picked up from 2018-01-30)

Thanks a lot for your help guys.