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from table a join table b on -- the primary key
where a.idNumber = b.idNumber

select abs(a.price - b.price) as difference

where a.date = (select max(Date) from table t1
      where t1.idNumber = a.idNumber)
and
     b.Date = (select max(Date) from table t2
     where t2.idNumber = b.idNumber)

select
      a.idNumber              -- Presuming you want
...
         where t1.idNumber = a.idNumber)
    and b.Date = (select max(Date) from table t2
         where t2.idNumber = b.idNumber)