isnull

In SQL, if I want to know whether an expression is NULL, I can use is null . But I don't understand how I can express is null in relational algebra. Can I use δ Field_Name=NULL(Table_Name) ? There is no NULL in relational algebra. In SQL the operators treat the value NULL specially, syntactically and semantically, differently than other values, usually by returning NULL when comparing two values when one of them is NULL. So "=" in a WHERE is not equality, it is an operator like equality that acts differently for NULL. So SQL WHERE is not the same operator as algebraic RESTRICT. If someone

Given the following: SELECT ISNULL('XY' + NULL, 'ABCDEFGHIJ') -- Outputs ABC (Why?) SELECT COALESCE('XY' + NULL, 'ABCDEFGHIJ') -- Outputs ABCDEFGHIJ Why are these statements returning different results? According to Microsoft documentation , for function: ISNULL(check_expression, replacement_value) replacement_value must be of a type that is implicitly convertible to the type of check_expression . Note that type for 'xy'+NULL is VARCHAR(3) . Because of this your string 'ABCDEFGHIJ' is cast to VARCHAR(3) and thus trimmed. It sounds strange why it is not VARCHAR(2) , but this is the way it is -

Possible Duplicate: What's the difference between is_null($var) and ($var === null)? PHP has two (that I know of, and three if you count isset() ) methods to determine if a value is null: is_null() and === null . I have heard, but not confirmed, that === null is faster, but in a code review someone strongly suggested that I use is_null() instead as it is specifically designed for the null-evaluation purpose. He also started talking about math or something. Anyway, the fact that is_null() is apparently slower also leads me to believe that it's doing more than === null does and is probably

I cannot get this code to work I must be missing something pretty simple. I am trying to check to see if a Cookie exists, if it does {do nothing} if it doesn't {create it}. I am testing the cookie by including an alert on a page. Basically I do not want the cookie to keep re-creating with a referral url, I am trying to grab only the FIRST referred URL. $(document).ready(function(){ if ($.cookie('bas_referral') == null ){ var ref = document.referrer.toLowerCase(); // set cookie var cookURL = $.cookie('bas_referral', ref, { expires: 1 }); } }); Displaying the current cookie contents: // get

I have a column called CODE in a MySQL table which can be NULL. Say I have some rows with CODE='C' which I want to ignore in my select result set. I can have either CODE=NULL or CODE!='C' in my result set. The following query does not return a row with CODE as NULL: SELECT * from TABLE where CODE!='C' But this query works as expected and I know it is the right way to do it. SELECT * from TABLE where CODE IS NULL OR CODE!='C' My question is why does having only CODE!='C' does not return rows where CODE=NULL? Definitely 'C' is not NULL. We are comparing no value to a character here. Can someone