How to subtract or add random number to mysql column in php?

Question

PHP

How to subtract or add a random number to a mysql column using php?
As an example, there is a column "a" with a value of 10, and how can I subtract a random number from these 10, specifically for column a.

Answer 1

if you take away no more than there is, then
update t set a = a - FLOOR( 1 + RAND( ) * a );