SQL腳本，有關相關子查詢

Question

我一直有困難，有一段腳本我創造的，並希望一些洞察我做錯了

問題：

使用相關子查詢返回每個供應商的發票，代表供應商最早的發票（具有最早日期的發票）。每行應包含以下四列：供應商名稱，發票號碼，發票日期和發票總額。

我的腳本：

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; 

目前，我的代碼是給太多的結果反饋，基本上是我無法確定這invoice_numbers只對最低日期可能被退回。

Answer 1

您需要限制加入到這些發票的特定供應商，並具有最早日期... 你不要;噸menmtion發票表是否具有替代PK ...如果它不「T，嘗試：

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; 

如果發票表中有一個替代PK，說InvoiceId，那就試試這個：

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; 

前者只使用一個子查詢，但後者更真實（更CLE arly表達）查詢意圖的意圖。

Answer 2

我對您的建議是不要在WHERE子句中使用子查詢，而要在內部JOIN中使用派生表。

SELECT 
    v.Vendor_Name, 
    i.Invoice_number as OLDEST_INVOICE, 
    i.Invoice_date, 
    i.invoice_total 
FROM Vendors v 
JOIN Invoices i ON v.vendor_id = i.vendor_id 
JOIN (SELECT 
     i2.vendor_id, 
     MIN(i2.invoice_date) AS min_invoice_date 
     FROM invoices i2 
     GROUP BY i2.vendor_id) minv 
    ON minv.vendor_id = v.vendor_id 
    AND minv.min_invoice_date = i.invoice_date 
ORDER BY i.Invoice_Date; 

Answer 3

根據您的平臺，這個可以寫多一點簡潔（並且很容易就能使用分析窗函數理解）：

http://my.safaribooksonline.com/book/databases/sql/0596004818/sql-functions/sqlnut2-chp-4-sect-3