有什么方法可以避免内部联接从客户表中获取客户名字和姓氏,我如何优化执行计划?
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
答案 0 :(得分:2)
有两种方法可以做到这一点。 一种方法是将所有非聚合列包括在列表中,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应该这样做):
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更好。要提高整体性能,您需要创建一个包含或覆盖索引。
您可以查看以下链接,了解所包含的索引:
答案 1 :(得分:1)
使用OVER()子句
尝试此选项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)
答案 2 :(得分:0)
尝试一下:
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