Saldo de débito de crédito de saldo aberto MySQL

select 
s.client_id,
s.date,
s.op_balance as Open_Balance,
s.credit,
s.debit,
s.balance
from
(
  select 
  t.client_id,
  t.date,
  t.credit,
  t.debit,
  @tot_credit := if(@prev_client = t.client_id, @tot_credit + t.credit,t.credit) as tot_cred,
  @tot_debit := if(@prev_client = t.client_id,@tot_debit + t.debit,t.debit) as tot_deb,
  @cur_bal := if(@prev_client = t.client_id, @tot_credit - @tot_debit,t.credit-t.debit) as balance,
  (@cur_bal + t.debit) - t.credit as op_balance,
  @prev_client := t.client_id
  from(
    select * from stock order by client_id,date
  )t,(select @prev_client:=0,@cur_bal:=0,@tot_credit:=0,@tot_debit:= 0,@open_balance:=0)r
)s