The Question :
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What does “unsigned” mean in MySQL and when should I use it?
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The Answer 1
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All integer types can have an optional
(nonstandard) attribute UNSIGNED.
Unsigned type can be used to permit
only nonnegative numbers in a column
or when you need a larger upper
numeric range for the column. For
example, if an INT column is UNSIGNED,
the size of the column’s range is the
same but its endpoints shift from
-2147483648 and 2147483647 up to 0 and 4294967295.
When do I use it ?
Ask yourself this question: Will this field ever contain a negative value?
If the answer is no, then you want an
UNSIGNED data type.
A common mistake is to use a primary key that is an auto-increment
INT starting at zero, yet the type is
SIGNED, in that case you’ll never touch any of the negative numbers and you are reducing the range of possible id’s to half.