im trying to get the previous transaction lines (ie bill --> po ) i need to get the correct transaction lines from each po

SELECT
transaction.ID as transaction_id,  
transactionLine.ID as transactionLine_id, 
transactionLine.item as item_id,
transactionLine.custcol6 as po_number,
PreviousTransactionLineLink.previousdoc  AS PO_ID
FROM transaction
JOIN  transactionLine ON transaction.ID = transactionLine.transaction
JOIN PreviousTransactionLineLink ON PreviousTransactionLineLink.nextline = transactionLine.id AND
PreviousTransactionLineLink.nextdoc = transactionLine.transaction
WHERE transaction.ID = 13636399 AND transactionLine.item  IS NOT NULL