title: Does "is NULL" in Where Clause Behave In A Special Way?
body: <p>I am trying to debug a stored procedure that I did not write. Specifically focused on the third line beginning with <code>AND (Citation_Main.User_Defined5</code>, if I set a subset of records' User_Defined5 column to NULL, using an update query, then the stored procedure writes an export file that I am expecting. However, if the User_Defined5 field is set to 'N', a file is not written. </p>
  
  <pre><code>DECLARE Cit_Select_Cursor CURSOR FOR SELECT Citation_Main.Number,Citation_Main.Issue_Date,
          Citation_Main.Issue_Time,Citation_Main.Officer_Comment1, Citation_Main.Officer_ID,
          Citation_Main.Officer_Name,Citation_Main.License_Plate,Citation_Main.License_State,
          Citation_Main.Location_Block,Citation_Main.Location_Street,Citation_Main.Location_Dir,
          Citation_Main.Location_Suffix,Citation_Main.Amount_Due,Citation_Main.Meter,
          Citation_Main.Make,Citation_Main.Color_Major,Citation_Main.Body_Style,
          Citation_Main.License_Type,Citation_Main.VIN,Citation_Activity.ChargeCode,Citation_Activity.DebitAmount
          ,Citation_Main.Officer_Comment2
          FROM Citation_Main --TTM_Bak
  --ADDED 03/12/2016 Inner Join and check for void and warning
  --      INNER JOIN Citation_Main On TTM_Bak.Number = Citation_Main.Number
          INNER JOIN Citation_Activity On Citation_Main.Citation_Key = Citation_Activity.Citation_Key
          Where --ExportViaDAT = 'N' And 
          Citation_Main.Void ='N' And Citation_Main.Warning ='N'
          AND (Citation_Main.User_Defined5 ='N' or Citation_Main.User_Defined5 ='' or Citation_Main.User_Defined5 is NULL)
          ORDER BY Citation_Main.Issue_Date DESC
  </code></pre>
  
  <p>I have been reading the suggested SO posts that appeared as I was entering this post, but could not see anything wrong with this part of the stored procedure. </p>
  
  <p>My question is, is there something inherently wrong with checking for NULL in this part of the stored procedure? </p>