Please note query #1, where both parameters are null. Then all rows are returned.

CREATE TABLE #temp (n NVARCHAR(1), f NVARCHAR(1))

INSERT INTO #temp VALUES ('a', 'b')
INSERT INTO #temp VALUES ('a', 'c')
INSERT INTO #temp VALUES ('b', 'c')
INSERT INTO #temp VALUES ('e', 'd')

DECLARE @n AS NVARCHAR(1)
DECLARE @f AS NVARCHAR(1)

SELECT * FROM #temp WHERE n = ISNULL(@n, n) AND f = ISNULL(@f, f)

SET @n = 'a'
SELECT * FROM #temp WHERE n = ISNULL(@n, n) AND f = ISNULL(@f, f)

SET @f = 'c'
SELECT * FROM #temp WHERE n = ISNULL(@n, n) AND f = ISNULL(@f, f)

SET @n = NULL
SELECT * FROM #temp WHERE n = ISNULL(@n, n) AND f = ISNULL(@f, f)

DROP TABLE #temp   

Results:

n    f
---- ----
a    b
a    c
b    c
e    d

n    f
---- ----
a    b
a    c

n    f
---- ----
a    c

n    f
---- ----
a    c
b    c

First, our setup. We wrote c# code in .Net 3.0, using a SQL Server 2005 Standard Edition database. Some actions in our business layer needed transactions, so we used TransactionScopes, developed just for this scenario. As we were using SQL Server 2005, which supports light-weight transactions (2000 does not), we shouldn’t ever need the DTC since we were using only one data source. But apparently, this wasn’t the case. So what was happening?

Here’s a snippet showing our situation:

using (TransactionScope ts = new TransactionScope())
{
    // First database connection, might happen in Business Logic -> Data Access Layer or anywhere
    using (SqlConnection conn = new SqlConnection( ... ))
    {
        // ...
    }

    // Second database connection, might happen in Business Logic -> Data Access Layer or anywhere. This is escalated to DTC. But why?
    using (SqlConnection conn = new SqlConnection( ... ))
    {
        // ...
    }

    ts.Complete();
}

When running this code in debug mode, reaching the second using-scope triggered an escalation to the DTC. But why? Apparently, we have two connections running at the same time, but that seems weird, since we’ve already left the using-scope so that connection should be closed in the dispose()-part of the using-scope.

It’s obvious when you think about it, really. When leaving the first using-scope, the connection should be closed. But that doesn’t happen, since we are in a transaction. The transaction scope needs to keep the connection open when we either commit or rollback. So when we reach the second using-scope, a second connection is opened and we are escalated to the DTC.

So, what can we do about this? There are at least two solutions:

• We overload the TransactionScope class with an implementation that re-uses the connection if a connection is already in use.
• Instead of calling new SqlConnection()... you can call a wrapper which checks if there already is one connection running and reuses that one.

If your are using SQL Server 2008 and Visual Studio 2008, this is implemented from the beginning and you don’t need any wrappers or overloads.

Recommended reading:

• http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=3083270&SiteID=1
• http://blogs.msdn.com/adonet/archive/2008/03/26/extending-lightweight-transactions-in-sqlclient.aspx
• http://blogs.msdn.com/dataaccess/archive/2006/02/14/532026.aspx

The behaviour is similar when using CAST and CONVERT. Then, the default is to limit the data to 30 characters.

Some examples of this:

DECLARE @string AS VARCHAR
SET @string = 'Lorem ipsum dolor sit amet, consectetur adipisicing elit'

SELECT
'Lorem ipsum dolor sit amet, consectetur adipisicing elit' AS [Original string],
LEN('Lorem ipsum dolor sit amet, consectetur adipisicing elit') AS [Length of original string],
@string AS [varchar],
CAST('Lorem ipsum dolor sit amet, consectetur adipisicing elit' AS VARCHAR) AS [Casted varchar],
LEN(CAST('Lorem ipsum dolor sit amet, consectetur adipisicing elit' AS VARCHAR)) AS [Length of casted varchar]
GO

DECLARE @string AS NVARCHAR
SET @string = N'Lorem ipsum dolor sit amet, consectetur adipisicing elit'

SELECT
N'Lorem ipsum dolor sit amet, consectetur adipisicing elit' AS [Original string],
LEN(N'Lorem ipsum dolor sit amet, consectetur adipisicing elit') AS [Length of original string],
@string AS [nvarchar],
CAST(N'Lorem ipsum dolor sit amet, consectetur adipisicing elit' AS NVARCHAR) AS [Casted nvarchar],
LEN(CAST(N'Lorem ipsum dolor sit amet, consectetur adipisicing elit' AS NVARCHAR)) AS [Length of casted nvarchar]

Results:

Original string                                          Length of original string varchar Casted varchar                 Length of casted varchar
-------------------------------------------------------- ------------------------- ------- ------------------------------ ------------------------
Lorem ipsum dolor sit amet, consectetur adipisicing elit 56                        L       Lorem ipsum dolor sit amet, co 30

(1 row(s) affected)

Original string                                          Length of original string nvarchar Casted nvarchar                Length of casted nvarchar
-------------------------------------------------------- ------------------------- -------- ------------------------------ -------------------------
Lorem ipsum dolor sit amet, consectetur adipisicing elit 56                        L        Lorem ipsum dolor sit amet, co 30

(1 row(s) affected)

Hello everyone!

SQL Tidbits is a place for me to ramble on about SQL and post tips on how to do weird or complicated stuff in SQL, T-SQL and SQL Server.

My name is Jonas Lincoln, I am currently employed at Netlight AS in Oslo, Norway. I’m usually located in Stockholm, Sweden, but I’m doing a wonderful year in Oslo starting up our business here.

I started using SQL Server 2000 back in 2002, while working on a large online game called Hattrick, one of the largest games in the world actually. As time passed, I got more and more involved and since we were on a tight budget, we had to do some really cool optimizations do get everything running. And that was when my interested in SQL started for real.

Now, I’m a senior consultant, mostly coding in C# and using the .Net framework.

If you wish to contact me, check out my personal page. ]]> http://sql-tidbits.lincoln.se/2008/12/10/introduction-to-sql-tidbits/feed/