The arithmetic operators in MySQL work only on numbers, not on strings. However, MySQL will convert a string into a number when in a numeric context if it can. If it can’t convert a particular string, it will return 0. Table B-1 lists the arithmetic operators allowed.
The minus sign may be used for subtracting numbers or for setting a
number to a negative. The DIV operator converts values
to integers and returns only integers. It doesn’t round fractions that
would be returned, but rather truncates them.