整数减去 null 得到是 null

select
  id,
  total,
  used,
  (total - used) as have
from test_table;

使用 ifnull 函数处理

select
  id,
  ifnull(total,0) as total,
  ifnull(used,0) as used,
  (ifnull(total,0) - ifnull(used,0)) as have
from test_table;