SELECT query with one or multiple IF statements in ABAP

I'm working on a web service. I have a select statement like this (it has much more fields and 2 more joins than this but to keep it simple):

SELECT acdoca~augbl,
       open_amount,    "I need to add value to this field based on the IF statement below
       acdoca~koart
FROM acdoca
      LEFT JOIN bseg ON acdoca~belnr EQ bseg~belnr
                      AND acdoca~gjahr EQ bseg~gjahr
                      AND acdoca~buzei EQ bseg~buzei
                      AND acdoca~rbukrs EQ bseg~bukrs
INTO table @it_acdoca.

I need to add this IF statement:

IF bseg-KOART = 'D' or bseg-KOART = 'K' 
    IF acdoca-AUGBL is INITIAL THEN open_amount = DMBTR
        ELSE open_amount = 0
    ENDIF.
ELSE open_amount = 0
ENDIF.

I need an answer that won't trash the performance of the service. Since ACDOCA and BSEG are some of the biggest database tables in SAP. I tried using LOOP after SELECT and then adding this IF inside that LOOP but that destroyed the performance totally.

This Is What IT_ACDOCA table should look like after

Hoping for a quick reply. Best regards.