How to Get the required List Item values by using Where Clause

Question

I have two tables named "TAX_RULES","BILL"
1. " Tax_Rules" (Sub_Head_Code,Tax_ID,Tax_Percentage)
{ While "Sub_Head_Code" field is unique,
Tax_ID describes that there are two kinds of Tax_IDs based on Tax_percentage which is 6% for some Sub_Head_Codes and 3.5% for the remainng Sub_Head_Codes. I have given Tax_ID 1 for 6% and Tax_ID 2 for Sub_Head_codes having 3.5%. }
2. "BILL" (Bill_ID,Sub_Head_Code,Tax_ID,Bill_Amount)
{While "Bill_ID" describes unique Bill Identification Number,
Sub_Head_Code and Tax_ID are behaving like composite foreign key from Tax_Rules table}
I have design form for both tables.
In BILL form i am trying to get values of Tax_ID from Tax_Rules table at run-time by using WHEN-NEW-FORM-INSTANCE.
For this whenever i select a Sub_Head_Code in Bill Form, all the Tax_IDs that i have recorded against each Sub_Head_Code which is either 1 or 2 against 6% and 3.5% respectively.
But I want that whenever i select a "Sub_Head_Code", Only that Tax_ID which is associated with that Sub_Head_Code ahould come in the LIST ITEM.
I have used WHERE clause but that is useless.
Following is the Code that i have applied in the WHE-NEW-ITEM-INSTANCE Trigger at FORM LEVEL
<CODE>
DECLARE
     rg_n1 VARCHAR2(40) :='TAX_ID';
     rg_idn1 RecordGroup;
     gc_idn1 GroupColumn;
     errcode NUMBER;
BEGIN
     CLEAR_LIST('BILL.TAX_ID');
     rg_idn1 := Find_Group(rg_n1);
IF      Id_Null(rg_idn1) then
          rg_idn1 := Create_Group(rg_n1);
     gc_idn1 := Add_Group_Column(rg_idn1,'EXPLAIN',CHAR_COLUMN,60);     
          gc_idn1 := Add_Group_Column(rg_idn1,'TAX_ID',CHAR_COLUMN,3);
     END IF;
     errcode := Populate_Group_With_Query(rg_idn1,'select TAX_ID,TAX_ID from TAX_RULES');
     POPULATE_LIST('BILL.TAX_ID',RG_IDn1);
END;
</CODE>

Answer 1

In the Tax Rules table, you state:
"Sub_Head_Code" field is unique
In the Bill table, you state:
Sub_Head_Code and Tax_ID are behaving like composite foreign key from Tax_Rules table
If the Sub_Head_Code is unique, then it's acting as a primary key, so Sub_Head_Code and Tax_ID in the Bill table are not behaving like a composite foreign key. The Sub_Head_Code is a foreign key, and the Tax_ID is irrelevant as far as keys are concerned.
It is not clear what it is you want to do.
If you want to display the Tax_IDs from the Bill table when you select a Sub_Head_Code from the Tax_Rules table, then change your query to:
select TAX_ID, TAX_ID from BILL where Sub_Head_Code = :Tax_Rules.Sub_Head_Code
If you want to display the Tax_IDs from the Tax_Rules table when you select a Sub_Head_Code from the Bill table, then change your query to:
select TAX_ID, TAX_ID from TAX_RULES where Sub_Head_Code = :Bill.Sub_Head_Code
If this is not what you want, then clarify what it is you want to do. Don't say:
"I want that whenever i select a "Sub_Head_Code", Only that Tax_ID which is associated with that Sub_Head_Code"
because it is not clear what tables you are referring to.