Let’s suppose for a minute that you want to retrieve the top x number of rows from any given query. Keep in mind that x is a value that your users can supply.
Sure, you could do string concatenation and EXEC sp_executesql ‘SELECT TOP ‘ + @count + ‘ x FROM y;’. But, then again, a vicious hacker could get all cute and decide to change your form variable to ‘; DECLARE @bad_var AS NVARCHAR(MAX); SET @bad_var = ””; SELECT @bad_var = @bad_var”DROP TABLE ” + name + ”;” FROM sys.tables; SET @bad_var = @bad_var + ”–”; EXEC sp_executesql @bad_var;’ Or something along those lines but probably infinitely more cunning.
Well, you could also accomplish this a different way using a subquery:
DECLARE @COUNT AS INT; SELECT @COUNT = 2; SELECT ContactId, FirstName, LastName, EmployeeId, AddressId, AddressLine1, AddressLine2, City, PostalCode FROM ( SELECT ROW_NUMBER() OVER (ORDER BY NEWID()) AS rownum, c.ContactId, c.FirstName, c.LastName, e.EmployeeId, a.AddressId, a.AddressLine1, a.AddressLine2, a.City, a.PostalCode FROM HumanResources.Employee AS e INNER JOIN Person.Contact AS c ON e.ContactId = c.ContactId INNER JOIN HumanResources.EmployeeAddress AS ea ON e.EmployeeId = ea.EmployeeId INNER JOIN Person.Address AS a ON ea.AddressId = a.AddressId ) AS x WHERE rownum <= @COUNT;
It’s not the prettiest thing on earth, but it works. Thanks to Rick Kierner over at rickdoes.net for asking me to help out with a similar query late last week.
And thanks for your help.
you can also use SET ROWCOUNT n, still works in 2008 but will be deprecated in the next version I believe, make sure that it is set to 0 after the query
declare @id int
select @id = 5
set rowcount @id
select * from sysobjects
order by newid()
set rowcount 0
I just found this:
DECLARE @Count int
SELECT @Count = 5
SELECT TOP (@Count),
c.ContactId,
c.FirstName,
c.LastName,
e.EmployeeId,
a.AddressId,
a.AddressLine1,
a.AddressLine2,
a.City,
a.PostalCode,
at.Name AS AddressType
FROM HumanResources.Employee AS e
INNER JOIN Person.Contact AS c
ON e.ContactId = c.ContactId
INNER JOIN HumanResources.EmployeeAddreess AS ea
ON e.EmployeeId = ea.EmployeeId
INNER JOIN Person.Address AS a
ON ea.AddressId = a.AddressId
INNER JOIN Person.AddressType AS at
ON a.AddressTypeId = at.AddressTypeId
Is this better/worse/same?
It actually turns out that the execution plans are almost exactly the same (in this case). Using TOP (@Count) is slightly more efficient within this scenario in AdventureWorks (the plan cost for my query is 0.412827 and for TOP it is 0.412769). The difference is made up by some additional operators and a filter that I’m using to replace the functionality of TOP via my use of ROW_NUMBER and the WHERE clause.
Which brings up the point that we’re always supposed to be using the parens with TOP and I just found that out this morning while reading Books Online.