A classic error is the attempt to use the equals operator codice_97 in combination with the keyword codice_1 to find rows with Nulls. According to the SQL standard this is an invalid syntax and shall lead to an error message or an exception. But most implementations accept the syntax and evaluate such expressions to codice_99. The consequence is that no rows are found regardless of whether rows with Nulls exist or not. The proposed way to retrieve rows with Nulls is the use of the predicate codice_9 instead of codice_101.