whats the difference between these Indexes?

Question

1- CREATE UNIQUE NONCLUSTERED INDEX idx_LatestPosts_1 ON dbo.LatestPosts(
Country ASC
,PostCateGory ASC
,PostID DESC
);

2- CREATE  NONCLUSTERED INDEX idx_LatestPosts_1 ON dbo.LatestPosts(Country)  InClude(PostCateGory)

as i understand we should Use Include statementright  only if both Counry and PostcateGory are Unique   its right??

there is any more differences?

Answer 1

Hi Greaso,

Unique indexes and included columns are 2 totally seperate concepts.

A unique index is a normal nonclustered index, wich maintains a unique constraint on the column or combination of columns that form the index key.

When you use included columns, instead of the key being stored at every level of the B-Tree structure, the included columns are only included in the index at the leaf level. This allows you to build covering index (indexes that cover every column in a query) without having hugh, inefficient indexes. It also allows you to get arounf the 900 byte/16 column limitation of index keys.

Both clustered and non-clustered indexes can be unique or non-unique. You can only include columns in a non-clustered index, but technically, in a clustered index, all columns are included anyway.

Hope this helps.

Pete

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Peter Carter http://sqlserverdownanddirty.blogspot.com/

Answer 2

Aslo please read this link

http://technet.microsoft.com/en-us/library/ms189607.aspx

VT

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