Running Totals with MySQL

I need to keep a running total (not an aggregate) based on data in two
columns e.g.
a   b   calculated_col
100    -10   90
50     10    150
-10    -20   120

the rows are selected on certain WHERE criteria and so the running total is
not fixed data. The running total is for display on the screen.
Doing this work in the language interfacing with MySQL is causing some odd
side effects so I was hoping to do the work on the MySQL side instead.

Kerry


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