Conceptually, NULL means “a missing unknown value” and it is treated somewhat differently from other values.
To test for NULL, use the IS NULL and IS NOT NULL operators,you cannot use arithmetic comparison operators such as =, <, or <> to test for NULL.
In MySQL, 0 or NULL means false and anything else means true. The default truth value from a boolean operation is 1.
A common error when working with NULL is to assume that it is not possible to insert a zero or an empty string into a column defined as NOT NULL, but this is not the case. These are in fact values, whereas NULL means “not having a value.”
mysql-NULL
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