I have a question or two about this ERD. 
1.  Regarding the Job table. It is a one to one relationship from Quotes.  The job number is assigned when the quote gets approved.  Is a Job table necessary?  If I use the Quote number for the job number there will be big spaces between the Job numbers.  I guess this would be a good option for a trigger that would import the Quote record into JObs?

COMPANY
CompanyID

QUOTES
QuoteID
CompanyID


QUOTES_Items
QI_ID
QuoteID
PO_ITemsID

JOBS
JobID
QuoteID

PURCHASE_ORDER
PO_ID
QI_ID
CHange_OrderID

CHANGE_ORDERS
Change_orderID


CHANGE_ORDERS_Items
CO_ITEMSID
PO_ID

INVOICES
InvoiceID
Quote_ItemsID

2.  Should PUrchase Orders have a relationship to Quotes_Items  or do you need a separate table for Invoice_items? 

3.  What about Invoices?  Do they also relate to Quotes_Items? What about PO_Line_Items?
3.  Quotes relates to JObs through QuoteID.  Does the reverse relationship work to Quotes_Items for Jobs_Items?

You wouldn't have a trigger to create Purchase Orders from quotes items you would need an import button on each item or something like that? I'm not clear on whether the Quotes Items is
Thanks,


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