SQL Rowcount vs Top

问题:

What is the difference between

Set Rowcount X

And

Select Top X *
From Z

in TSQL?

回答1:

Top can do a few more things for you. For one, you can specify a percentage, instead of an integer. You can also handle situations where ties in column values occur.

http://technet.microsoft.com/en-us/library/ms189463.aspx

回答2:

The main difference is that top will only effect the query you are running while set rowcount will persist with the connection and apply to all queries executed within that connection.

回答3:

In older versions of SQL Server (2005 and earlier I am not sure about 2008) you could not use a variable in a top statement so:

declare @rc int

set @rc=10

select top @rc * from myTable --Wont work

set rowcount @rc
select * from myTable --Will work

回答4:

2008 and above allows

declare @rc int

set @rc=10000

select top (@rc) * from myTable --will now work

but only if you use () this can be usefull to use hints like:

OPTION ( OPTIMIZE FOR (@rc= 15) )

at the end allowing to select everithing but optimize for a more common case