how to subtract of two numbers in mysql
select sum(s0.w - s1.w) as total_weight_loss
from (select idMember, max(weight) as w from Measurements m
where m.date = (select min(date) from Measurements n
where m.idMember = n.idMember)
group by idMember) s0
join (select idMember, min(weight) as w from Measurements m
where m.date = (select max(date) from Measurements n
where m.idMember = n.idMember)
group by idMember) s1
on s0.idMember = s1.idMember