Conditional Select - Part 2 - oracle

Question

I have the following situation within a PL/SLQ function. Depending of existing table field value I might run a different select. Specifically: I can have multiple rows for a particular BILL CODE (PINGPONG) where I would only need to get the SYS_FIELD value. This field has to be fetched only once according to following condition: If fields prep_seq_num=0 and primary_ind=0 then just get this row sys_field value straightaway and do not take care of other possible prep_seq_num and primary_ind values different from 0. If that rows is not existing, fetch the sys_field value from prep_seq_num!=0 and primary_ind=1. For both case only one instance/row must be possible So in the first case I should run:
SELECT SYS_FIELD
INTO v_start_of_invoice
FROM BILL
WHERE TRACKING_ID = v_previous_trackingID
AND BSCO_CODE_ID = 'PINGPONG'
AND CHRG_ACCT_ID = v_ACCT_ID
AND PREP_SEQ_NUM = 0 -- maybe not needed here
AND ITEM_CAT_CODE_ID=1
AND PARTITION_KEY = v_prev_partition
AND SUBPARTITION_KEY = v_prev_subpartition
AND PRIMARY_IND=0;
In the second case
SELECT SYS_FIELD
INTO v_start_of_invoice
FROM BILL
WHERE TRACKING_ID = v_previous_trackingID
AND BILL_CODE_ID = 'PINGPONG'
AND ITEM_CAT_CODE_ID in ('5' , '-100')
AND PARTITION_KEY = v_prev_partition
AND SUBPARTITION_KEY = v_prev_subpartition
AND PRIMARY_IND=1;
Not sure I'm making it very complicated, but still I 'd need to know whether IF THEN ELSE or CASE or whatever should be used and how.

Answer 1

You can utilize an implicit cursor for loop.
for rec IN ( <your query that gives all records)
LOOP
--compare the value of this variable and decide in the IF condition.
IF rec.prep_seq_num=0 and rec.primary_ind=0
THEN
-- write the query to get the sys_field
ELSE
-- write alternative query.
END IF;
END LOOP;