data-konzept

'Any SQL Server database developer knows that NULL does not mean 0 or an empty string, it means that the value is unknown. The result of a straightforward comparison with NULL is unknown, so special instructions are required (IS NULL, COALESCE, NULLIF, etc.) to get the desired correct result in our T-SQL code. However, there are other (less popular) subtleties related to NULL, which must be understood to write bug free code. In this tip we will discuss some of them.'...

https://www.mssqltips.com/sqlservertip/4082/some-tricky-situations-when-working-with-sql-server-nulls/