如何避免額外的內部連接來優化此查詢？

Question

有沒有什麼辦法可以避免內部連接從客戶表中獲取客戶名和姓，以及如何優化執行計劃？

SELECT c1.firstname, c1.lastname, t.* 
FROM customer c1 
INNER JOIN 
    (select c.Customerid ,count(si.CustomerID) as 'No Of Orders' 
    from Customer c 
    LEFT OUTER join SalesInvoice si ON si.CustomerID = c.CustomerID 
    GROUP by c.CustomerID) as t 
ON c1.CustomerID = t.Customerid 

Answer 1

有兩種方法可以做到這一點。 一種方式是通過列表如下，以包括所有非聚合列入組：

SELECT 
    c.firstname, 
    c.lastname, 
    c.CustomerId, 
    count(si.CustomerID) as 'No Of Orders' 
FROM 
    customer c 
LEFT OUTER JOIN 
    SalesInvoice si ON si.CustomerID = c.CustomerID 
GROUP BY 
    c.CustomerID, 
    c.firstname, 
    c.lastname 

另一種方法來做到這一點，將在選擇列表中使用集合函數（MIN或MAX應該做的） ：

SELECT 
    MIN(c.firstname) as firstname, 
    MIN(c.lastname) as lastname, 
    c.CustomerId, 
    count(si.CustomerID) as 'No Of Orders' 
FROM 
    customer c 
LEFT OUTER JOIN 
    SalesInvoice si ON si.CustomerID = c.CustomerID 
GROUP BY 
    c.CustomerID 

我覺得第一個查詢在性能上可能會更好。爲了提高整體表現，您需要創建一個包含或覆蓋索引。

你可以看看以下鏈接給你列入索引的想法：

1. Why use the INCLUDE clause when creating an index?
2. http://msdn.microsoft.com/en-IN/library/ms190806.aspx

Answer 2

嘗試一個：

SELECT 
    c.firstname, 
    c.lastname, 
    c.CustomerId, 
    count(si.CustomerID) as 'No Of Orders' 
FROM 
    customer c 
LEFT OUTER JOIN 
    SalesInvoice si ON si.CustomerID = c.CustomerID 
GROUP BY 
    c.CustomerID, 
    c.firstname, 
    c.lastname 

Answer 3

嘗試使用此選項與OVER()條款

SELECT c1.Customerid, c1.firstname, c1.lastname, 
     COUNT(si.CustomerID) OVER(PARTITION BY c1.CustomerID) AS 'No Of Orders' 
FROM customer c1 LEFT JOIN SalesInvoice si ON si.CustomerID = c1.CustomerID 

爲了提高性能，你需要這個指數

CREATE INDEX ix_Customerid_Customer ON Customer(Customerid) INCLUDE(firstname, lastname)