Opened by Gast at 2009-12-26 11:03
Mit "warum gibt es keinen UNKNOWN Status?" wollte ich Bezug nehmen auf die von mir verlinkte Seite; ich dachte, damit wird es verständlicher, was ich meine.
QuoteIn relational operations, unlike normal binary, there are THREE states that a operation can result in, TRUE, FALSE, and UNKNOWN. [...]
The UNKNOWN state occurs for any logical operation where the result cannot be evaluated as either TRUE or FALSE, and is connected with the NULL concept.
A NULL in a SQL table, can be regarded as a placeholder for missing information, its not a VALUE as such, just a marker to indicate the lack of information.