types – What does `unsigned` in MySQL mean and when to use it?

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What does “unsigned” mean in MySQL and when should I use it?

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MySQL says:

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.

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