CREATE TABLE T
(
A INT,
B INT,
C INT ,
PRIMARY KEY CLUSTERED (B DESC, C DESC)
)
/*Implicitly adds B DESC, C DESC to end of key*/
CREATE NONCLUSTERED INDEX ix1 ON T(A ASC)
/*No sort operation*/
SELECT *
FROM T
ORDER BY A ASC,B DESC, C DESC
/*
But the avove index won't be able to seek into A,C
and will need a residual predicate after seeking into A.
For the following query
*/
SELECT *
FROM T
WHERE A=1 AND C > 4
ORDER BY C ASC, B DESC
/*This index explicitly controlling the key column position
and direction would be better*/
CREATE NONCLUSTERED INDEX ix2 ON T(A ASC, C ASC, B DESC)
在 SQL Server 中，聚集索引键列始终添加到非聚集索引中以充当行定位器（参考：关于非聚集索引键的更多信息）。
对于声明为唯一的 NCI，它们将作为包含列添加，否则将添加到键的末尾。
如果默认位置不适合您的查询，您可能需要显式添加列。例如如果你想控制
ASC/DESC方向或者你想控制键列在索引中的位置。