The database in your example does not state, either explicitly or implicitly, that Amount is 10, therefore this is false. It also does not state that Amount is not 10, therefore this is also false.
In the real world, you cannot have an amount that is at the same time not 10 and not not 10. It has to be one of both. The amount is perhaps unknown (if that is what you mean to express by having a "NULL"-value in this field). But even if it is unknown, it has to be 10 or not 10 (but we do not know which one). So a query that asks for all rows where the amount is 10 or the amount is not 10, must return all rows. SQL does not return the rows where the amount is "NULL" and that is where it is flawed and gives a wrong result. The relational model cannot work with a "NULL"-value. If you want to have a value for "unknown", you must have TWO values for unknown (as per CODD).

CountZero

A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

In reply to Re^5: DBI: passing undef as an argument by CountZero
in thread DBI: passing undef as an argument by fws

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