SQL - 拉客戶端（結果集中每行有1個客戶端）以及這些客戶端的最近訂單日期

Question

下面是一些SQL以拉取客戶端列表。正如你所看到的，我添加了由「 - ADDED」註釋的行 - 一個新字段，一個連接和一個order by。我試圖在結果集中顯示每行一個客戶端，但對於每個客戶端，我想顯示最近的訂單。什麼是最好的方法來做到這一點？我應該在我的JOIN中放置一個位置還是使用TOP 1？我想有幾種方法，但我想盡可能提高效率。

SELECT 
    CONVERT(NVARCHAR(255),client.identifyingnumber) As [Client ID], 
    CONVERT(VARCHAR,client.name) As [Client Name], 
    client.clientid As [Id], 
    CONVERT(CHAR(10),[ecom_order].[order_date],101) -- ADDED 
FROM 
    client 
    JOIN clientstatus ON client.clientstatusid = clientstatus.clientstatusid 
    JOIN clienttype ON clienttype.clienttypeid = client.clienttypeid 
    -- keep left outer join since some clients won't have any orders, thus no last order 
    LEFT OUTER JOIN [ecom_order] ON client.clientid = [ecom_order].clientid -- ADDED 
WHERE 
    client.name LIKE '%_%' 
    AND 
    client.name <= (
     SELECT MAX(maxsubquery.ordercolumn) FROM (
      SELECT 
       TOP 10 client.name AS ordercolumn 
      FROM 
       client 
       JOIN clientstatus ON client.clientstatusid = clientstatus.clientstatusid 
       JOIN clienttype ON clienttype.clienttypeid = client.clienttypeid 
      WHERE 
       client.name LIKE '%_%' 
      ORDER BY client.name 
     ) 
     AS maxsubquery) 
ORDER BY 
    client.name, 
    [ecom_order].order_date DESC -- ADDED 

測試數據結果集示例：

Answer 1

試試這個

SELECT 
     CONVERT(NVARCHAR(255),client.identifyingnumber) As [Client ID], 
     CONVERT(VARCHAR,client.name) As [Client Name], 
     client.clientid As [Id], 
     -- Change to get latest 
     CONVERT(CHAR(10),Max([ecom_order].[order_date]),101)  
    FROM client 
     JOIN clientstatus ON client.clientstatusid = clientstatus.clientstatusid 
     JOIN clienttype ON clienttype.clienttypeid = client.clienttypeid 
     -- keep left outer join since some clients won't have any orders, 
    -- thus no last order 
     LEFT OUTER JOIN [ecom_order] ON client.clientid = [ecom_order].clientid 
    WHERE 
     client.name LIKE '%_%' 
     AND 
     client.name <= (
      SELECT MAX(maxsubquery.ordercolumn) FROM (
       SELECT 
        TOP 10 client.name AS ordercolumn 
       FROM 
        client 
        JOIN clientstatus ON client.clientstatusid = clientstatus.clientstatusid 
        JOIN clienttype ON clienttype.clienttypeid = client.clienttypeid 
       WHERE 
        client.name LIKE '%_%' 
       ORDER BY client.name 
      ) 
      AS maxsubquery) 
    GROUP BY client.identifyingnumber,client.name,client.client_id 
    ORDER BY 
     client.name