Sample SQL Double Join 
Use in Manu Product Sales Report

SELECT transact.recnum, catalog.manu_name, transact.prodline, transact.item_no, transact.inv_no, transact.item_no, catalog.manu_name, manufac.account 
FROM transact 
INNER JOIN catalog ON transact.item_no =catalog.item_no  
INNER JOIN  manufac ON manufac.company = catalog.manu_name  
WHERE transact.doc_type = 'INVOICE' 
ORDER by catalog.manu_name, transact.prodline,  transact.item_no