data persistence -- help with some logic

[jite eghagha]

Hi, i attached a picture of my table.

You'll notice, that on insert or update, a row can have a datetime value or not.

A player (player_id) earns $1 per second, so long as that row contains a datetime value.

If a row that once contained a datetime vaule is updated, or if that row is deleted, so that it no longer contains a datetime value, how do i ensure that the players total earning are not lost.

further more, a player may insert a new row at any time that could add more $ per second to the total earnings.

I know i need a second table for the total earnings, but how should i go about regularly updating the total earnings.

I'd appreciate any thoughts.