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I'm new to SQL concepts, while studying NULL expression I wonder why NULL can't match with NULL can anyone tell me a real world example to simply this concept?
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NULL is false, even if that NULL is from another column.
By default, a column can hold NULL values. The NOT NULL constraint enforces a column to NOT accept NULL values. This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field.
The result of a join of null with any other value is null. Because null values represent unknown or inapplicable values, Transact-SQL has no basis to match one unknown value to another. You can detect the presence of null values in a column from one of the tables being joined only by using an outer join.
NULL indicates an absence of a value. The designers of SQL decided that it made sense that, when asked whether A (for which we do not know its value) and B (for which we do not know its value) are equal, the answer must be UNKNOWN - they might be equal, they might not be. We do not have adequate information to decide either way.
You might want to read up on Three valued logic - the possible results of any comparison in SQL are TRUE, FALSE and UNKNOWN (mysql treats UNKNOWN and NULL as synonymous. Not all RDBMSs do)
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Rule : Not even a NULL can be equal to NULL.
A Non-Technical aspect
If you ask two girls, how old they are? may be you would hear them to refuse to answer your question, Both girls are giving you NULL as age and this doesn't mean both have similar age. So there is nothing can be equal to null.
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