如何避免额外的内部联接来优化此查询?

有两种方法可以做到这一点。 一种方法是将所有非聚合列按列表包括到组中,如下所示:

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

另一种方法是在选择列表中使用聚合函数(min或max should do):

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

我认为第一个查询的性能可能会更好。为了提高整体性能,您需要创建一个包含索引或覆盖索引。

您可以查看以下链接,了解包含索引:

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

使用尝试此选项 OVER() 条款

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

为了提高性能,您需要此索引

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

试试这个:

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