how to set a row's value from a certain row's value? I have a MySQL table named "activities" with fields id(int), tstamp(timestamp), amount(decimal) balance(decimal). Decimal fields hold money values.

id   tstamp                   amount   balance
----------------------------------------------
 1   2013-03-18 00:00:10       57.00      0.00
 2   2013-03-18 00:00:11       13.05      0.00
 3   2013-03-18 00:00:12      110.00      0.00
 4   2013-03-18 00:00:13       23.50      0.00
 5   2013-03-18 00:00:14       35.44      0.00
 6   2013-03-18 00:00:15       76.00      0.00
 7   2013-03-18 00:00:16       34.74      0.00
 8   2013-03-18 00:00:17      120.47      0.00
 9   2013-03-18 00:00:18       35.00      0.00
10   2013-03-18 00:00:09       46.00      0.00


so balance fields' values must be like that:
current row's balance = CHRONOLOGICALLY previous row's balance + current row's amount.

Notice last row's tstamp value is smaller than first row's tstamp value. so when I say previous row I do not mean current id minus 1. So highest balance value must be at row #9.

And the problem is how to update all balances with chronogically previous row's balance value + current row's amount value?
Assuming that the tstamp has a UNIQUE constraint: 

UPDATE activities AS a
  JOIN
  ( SELECT cur.tstamp,
           SUM(prev.amount) AS balance 
    FROM activities AS cur
      JOIN activities AS prev
        ON prev.tstamp &lt;= cur.tstamp
    GROUP BY cur.tstamp
  ) AS p
  ON p.tstamp = a.tstamp
SET a.balance = p.balance ;


Tested: SQL-Fiddle



MySQL has also a feature to use ORDER BY with an UPDATE, which you can combine with the use of variables:

SET @b := 0 ;
UPDATE activities
SET balance = (@b := amount + @b)
ORDER BY tstamp ;


Tested: SQL-Fiddle