What Happens If NULL Values Are Involved in Expressions

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Q

What Happens If NULL Values Are Involved in Expressions? - MySQL FAQs - Introduction to SQL Basics

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A

If NULL values are used in expressions, the resulting values will be NULL values. In other words:

Arithmetic expressions with NULL values result NULL values.
Comparison expressions with NULL values result NULL values.
Logical expressions with NULL values result NULL values.

The tutorial exercise shows you some interesting examples:


SELECT NULL + NULL FROM DUAL;
   NULL
  
SELECT NULL + 7 FROM DUAL;
   NULL

SELECT NULL * 7 FROM DUAL;
   NULL

SELECT NULL = NULL FROM DUAL;
   NULL

SELECT 0 < NULL FROM DUAL;
   NULL

SELECT '' > NULL FROM DUAL;
   NULL

SELECT NULL AND TRUE FROM DUAL;
   NULL

SELECT NULL OR TRUE FROM DUAL;
   1
   -- This is contradicting against the rules!

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