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PHP and SQL are entirely different languages. Their definitions are not linked in any way.

For example, an optional integer field. No integer value can be used to indicate a lack of a value, because that integer could potentially be a value in itself, including zero. A value is required...

Yes, when you wish to have a value that represents a lack of a value: for example, when you need to distinguish between an empty string or zero and an actual lack of a value. In relationships, as...

As well as all possible values of a column type, including values such as zero or an empty string (sometimes confusingly referred to as a 'null string' for C reasons) there is also an additional...