Weird calculations

I've found some problem which suprised me a little bit. My PHP script is executing this SQL query: mysql> SELECT indexes_values.value FROM indexes_values, indexes_names WHERE locked=0 AND indexes_values.nameid=indexes_names.ID AND indexes_names.name='MISC-IND' AND indexes_names.market='JSX' ORDER BY date DESC LIMIT 2; +----------+ | value | +----------+ | 103.7916 | | 102.8137 | +----------+ 2 rows in set (0.00 sec) where VALUE is decimal(8,4), and this part of the script is processing result: [...] if (mysql_numrows($result)>0) { $rrow=mysql_fetch_row($result); $v1 = $rrow[0]; $rrow=mysql_fetch_row($result); $v2 = $rrow[0]; } $curr_value = $v1; $change_value = ($v1-$v2); [...] where non of the values such as $v1 or $change_value has been initialised before, and finally executing echo "$changed_value"; what shows result "0.97790000000001". Well from what I know result of 103.7916-102.8137 should always be 0.9779, right ? So what is the problem ? Certailny if I will change echo "$changed_value"; to printf("%8.4f",$change_value); everything will be fine, but I am just wondering why I get this wrong result in first place ? I've found that the problem exists only for this particulal calculation, all other calculations in the database does not create any problem. I will also notice that echo "($v1,$v2)"; correctly displays "(103.7916,102.8137)". Best Regards, Marek Bialoglowy (ultor@systemintegra.com) - IT Security Researcher PGPkey: http://www.systemintegra.com/pgp/ultor.asc | ID: 0x4B36656E JOB: (CTO) System Integra | JKT, Indonesia | Timezone: JAVT, GMT +7