while select statement does not select all records

Hi,

I've added one checkbox on my purchTable form. This checkbox is on both purchTable header and purchLines. My question is when i select this checkbox from my header i.e. purchTable and it should get selected for all my purchLine checkboxes. Following below is my code:

 while select purchLine
        where purchTable.PurchId == purchLine.PurchId
    {
    if (purchTable.fixedasset == true)
    {
    purchLine.fixedasset = true;

}

}

the problem in the above syntax is it is selecting only the last line of the purchLine record. If i put in my select as

"While select firstonly100 purchLine"  then it is selecting the first record of my purchLine.

plz help me.