SQL脚本，有关相关子查询

Question

我一直有困难，有一段脚本我创造的，并希望一些洞察我做错了

问题：

使用相关子查询返回每个供应商的发票，代表供应商最早的发票（具有最早日期的发票）。每行应包含以下四列：供应商名称，发票号码，发票日期和发票总额。

我的脚本：

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，我的代码是给太多的结果反馈，基本上是我无法确定这invoice_numbers只对最低日期可能被退回。

Answer 1

您需要限制加入到这些发票的特定供应商，并具有最早日期... 你不要;吨menmtion发票表是否具有替代PK ...如果它不“T，尝试：

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; 

如果发票表中有一个替代PK，说InvoiceId，那就试试这个：

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; 

前者只使用一个子查询，但后者更真实（更CLE arly表达）查询意图的意图。

Answer 2

我对您的建议是不要在WHERE子句中使用子查询，而要在内部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

根据您的平台，这个可以写多一点简洁（并且很容易就能使用分析窗函数理解）：

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