Text fields are never null, that means even if you didn't provide any value ISNULL() function takes empty as a value. so using ISNULL() with a text field always returns false. For example, the formula field==== IF(ISNULL(new__c) 1, 0) is always zero regardless of the value in the New field.
For text fields, use the ISBLANK function instead. IF(ISBLANK(new__c) 1, 0) true if if has a value IF(ISBLANK(new__c) 1, 0) false if it don't have a value
As @Johan Liljegren said, the answer is right in the Online Help>
https://na2.salesforce.com/help/doc/user_ed.jsp?section=help&target=customize_functions.htm&loc=help&hash=ISNULL
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This is very well described in the Help & Training area. Simply search for isblank or isnull to find it.
To put it short, use ISBLANK(). =)
As @Johan Liljegren said, the answer is right in the Online Help>
https://na2.salesforce.com/help/doc/user_ed.jsp?section=help&target=customize_functions.htm&loc=help&hash=ISNULL
so using ISNULL() with a text field always returns false.
For example, the formula field==== IF(ISNULL(new__c) 1, 0) is always zero regardless of the value in the New field.
For text fields, use the ISBLANK function instead.
IF(ISBLANK(new__c) 1, 0) true if if has a value
IF(ISBLANK(new__c) 1, 0) false if it don't have a value