Null Values

   Undefined values, or "undef", can be used to indicate null values. 
+ However, care must
   be taken in the particular case of trying to use null values to qua
+lify a "SELECT"
   statement. Consider:

     SELECT description FROM products WHERE product_code = ?

   Binding an "undef" (NULL) to the placeholder will not select rows
   which have a NULL "product_code"! Refer to the SQL manual for your 
+database
   engine or any SQL book for the reasons for this.  To explicitly sel
+ect
   NULLs you have to say ""WHERE product_code IS NULL"" and to make th
+at
   general you have to say:

     ... WHERE (product_code = ? OR (? IS NULL AND product_code IS NUL
+L))

   and bind the same value to both placeholders. Sadly, that more gene
+ral syntax
   doesn't work for Sybase and MS SQL Server. However on those two ser
+vers
   the original ""product_code = ?"" syntax works for binding nulls.
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