NULL in the Oracle Database — Is it special, or strange, or just wrong?

Wrong. NULL means that data is missing. A blank string is not missing data, it is actually data that says that the string is empty. By making an empty string NULL, it causes havoc with query joins, because an inner join against a NULL value will never return the rows. But you may want to join empty string values.

Also, if you add NULL to an IN(...) clause, it will return nothing at all. If you do a subselect that returns an empty string, then this will stop the query from working.

More traps for young players. Can't believe Oracle hasn't fixed this yet!