Syntax:
ABS(X)

Returns the absolute value of X, or NULL if X is NULL.

The result type is derived from the argument type. An implication of
this is that ABS(-9223372036854775808) produces an error because the
result cannot be stored in a signed BIGINT value.

URL: https://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html

