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SQL Server FAQ - What Are NULL Values

By: FYIcenter.com

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What Are NULL Values?

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()

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1. What Are NULL Values?
2. How To Assign NULL Values to Variables or Columns?
3. What Happens If NULL Values Are Involved in Arithmetic Operations?
4. What Happens If NULL Values Are Involved in String Operations?
5. What Happens If NULL Values Are Involved in Datetime Operations?
6. What Happens If NULL Values Are Involved in Bitwise Operations?
7. What Happens If NULL Values Are Involved in Comparison Operations?
8. What Happens If NULL Values Are Involved in Boolean Operations?
9. How To Test NULL Values Properly?
10. How To Replace NULL Values in Expressions using ISNULL()?
11. How To Replace Given Values with NULL using NULLIF()?