Question

I have been having difficulties with a piece of script I am creating, and was hoping for some insight into what I am doing wrong

QUESTION:

use a correlated subquery to return invoice(s) for each vendor, representing the vendor’s oldest invoice (the one with the earliest date). Each row should include these four columns: vendor name, invoice number, invoice date, and invoice total.

MY SCRIPT:

SELECT DISTINCT Vendor_Name,
    Invoice_number AS OLDEST_INVOICE,
    Invoice_date,
    invoice_total
FROM Vendors v
JOIN Invoices i ON v.vendor_id = i.vendor_id
WHERE invoice_date IN (
        SELECT DISTINCT MIN(invoice_date)
        FROM invoices i
        JOIN vendors v ON i.vendor_id = v.vendor_id
        GROUP BY v.vendor_name
        )
ORDER BY Invoice_Date;

Currently, my code is giving too many results back, essentially I am having trouble establishing that invoice_numbers are only to be returned for the lowest date possible.

Answer 1

You need to restrict the join to those invoices for the specific vendor, AND that has the earliest date…
You don;t menmtion whether the Invoice table has a surrogate PK… If it doesn’t, try:

 SELECT 
    v.Vendor_Name,  
    i.Invoice_number as OLDEST_INVOICE,  
    i.Invoice_date,  
    i.invoice_total  
 FROM Vendors v  
 JOIN Invoices i 
     ON i.vendor_id = v.vendor_id
        And i.invoice_date =
            (Select MIN(invoice_date)
             From Invoices
             Where vendor_id = v.Vendor_Id)  
ORDER BY i.Invoice_Date;

If the invoice table has a surrogate PK, say InvoiceId, then try this:

 SELECT 
    v.Vendor_Name,  
    i.Invoice_number as OLDEST_INVOICE,  
    i.Invoice_date,  
    i.invoice_total  
 FROM Vendors v  
    JOIN Invoices i 
       ON i.InvoiceId = 
          (Select InvoiceId
           From Invoices
           Where vendor_id = v.Vendor_Id
              And Invoice_Date =
                   (Select Min(Invoice_Date)
                    From Invoices
                    vendor_id = v.Vendor_Id))
ORDER BY i.Invoice_Date;

The former only uses one subquery, but the latter is truer (more clearly expresses) the intent of the query’s intent.