What Are NULL Values in SQL Server Transact-SQL?

A NULL value is a special value that represents an unknown value. SQL Server supports NULL values with the following features:

• All data types used for table columns support NULL values. In another word, NULL values can be stored in database tables.
• Individual table columns may be defined to not allow NULL values. In this case, you can not assign NULL values to those columns.
• "NULL" is a keyword that represent a NULL value in expressions.
• NULL values can be used directly in SET (assignment) statements.
• If NULL values are involved in an arithmetic operation, the result will be a numeric NULL.
• If NULL values are involved in a string operation, the result will be a string NULL.
• If NULL values are involved in a datetime operation, the result will be a datetime NULL.
• If NULL values are involved in a bitwise operation, the result will be a binary NULL.
• If NULL values are involved in a comparison operation, the result will be a Boolean NULL.
• If NULL values are involved in a Boolean operation, the result could be TRUE, FALSE, or NULL.
• To test NULL values, you need to use two special operators, IS NULL and IS NOT NULL.
• Special functions are available to handle NULL values, like ISNULL(), and NULLIF()

⇒Working with NULL Values in SQL Server Transact-SQL

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