Sunday, November 14, 2010

ORDER BY Enhancements in SQL Server Denali

Today I am going to talk about one new feature that is been introduced in the SQL Server Denali.   And that is:  one interesting enhancement in ORDER BY clause.  Now with ORDER BY clause you can also specify the OFFSET and FETCH options. 

From Books Online of Denali:
OFFSET: Specifies the number of rows to skip before it starts to return rows from the query expression. The value can be an integer constant or expression that is greater than or equal to zero.

FETCH: Specifies the number of rows to return after the OFFSET clause has been processed. The value can be an integer constant or expression that is greater than or equal to one.

This is how the new syntax of ORDER BY clause looks like in SQL Server Denali:
------------------------------------------------------------------------------------------------------
ORDER BY order_by_expression
    [ COLLATE collation_name ]
    [ ASC | DESC ]
    [ ,...n ]
[ <offset_fetch> ]
<offset_fetch> ::=
{
    OFFSET { integer_constant | offset_row_count_expression } { ROW | ROWS }
    [
      FETCH { FIRST | NEXT } {integer_constant | fetch_row_count_expression } { ROW | ROWS } ONLY
    ]
}

------------------------------------------------------------------------------------------------------

Take a look at following example:

------------------------------------------------------------------------------------------------------
USE AdventureWorks
GO
SELECT
VendorID
    ,Name
FROM Purchasing.Vendor
ORDER BY VendorID OFFSET 5 ROWS
GO

------------------------------------------------------------------------------------------------------

Output:

Offset

In the example I wrote 5 as OFFSET.  You can see in the output 1st five rows are skipped and we got VendorIds starting from 6.  Here SQL Server 1st orders the data on the Column specified in ORDER BY clause(i.e. VendorID).  The next query uses the clause OFFSET 5 ROWS to skip the first 5 rows and return all remaining rows.

Now lets see the example of FETCH:

------------------------------------------------------------------------------------------------------
USE AdventureWorks
GO
SELECT
VendorID
    ,Name
FROM Purchasing.Vendor
ORDER BY VendorID OFFSET 5 ROWS
    FETCH NEXT
3 ROWS ONLY
GO

------------------------------------------------------------------------------------------------------

Output:

FETCH

As you can SQL Server has fetched only 3 rows and that also after skipping 1st five rows.  That is because I specified 5 as OFFSET and 3 as FETCH NEXT.  Here FETCH NEXT 3 ROWS ONLY to limit the rows returned to 3 rows from the sorted result set.

Also:
1. offset_row_count_expression can be a variable, parameter, or constant scalar subquery. When a subquery is used, it cannot reference any columns defined in the outer query scope. That is, it cannot be correlated with the outer query.

2. ROW and ROWS are synonyms and are provided for ANSI compatibility.

3. In query execution plans, the offset row count value is displayed in the Offset attribute of the TOP query operator.

You can find all this information and more on: ORDER BY Clause (Transact-SQL)

Thanks
Mangal

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