What is the difference between select count(*) and select count(any_non_null_column)?
COUNT(*) will include NULLS
COUNT(column_or_expression) won't.


This means COUNT(any_non_null_column) will give the same as COUNT(*) of course because there are no NULL values to cause differences.

Generally, COUNT(*) should be better because any index can be used because COUNT(column_or_expression) may not be indexed or SARGable

From ANSI-92 (look for "Scalar expressions   125")


  Case:
  
  a) If COUNT(*) is specified, then the result is the cardinality
                of T.
  
  b) Otherwise, let TX be the single-column table that is the
                result of applying the &lt;value expression&gt; to each row of T
                and eliminating null values. If one or more null values are
                eliminated, then a completion condition is raised: warning-
                null value eliminated in set function.


The same rules apply to SQL Server and Sybase too at least

Note: COUNT(1) is the same as COUNT(*) because 1 is a non-nullable expression.