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reducing the count for an item in a column

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I am trying to reduce a number in my column by 1. I need to do this as on my php script when someone buys a product the amount should reduce by 1. I have tried the following:

update [i]tablename[/i] set amount = 'amount' - 1 where ID = '005';

update [i]tablename[/i] set amount = 'amount - 1' where ID = '005';

The first statement doesnt changes the amount to -1. The second statement sets the amount to 0. What statement should i be using in mysql?? as i then need to use the statement in my php page.

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Using single quotes (') makes MySQL think it's a string. Just treat it like a column name both times.

[code]update tablename set amount = amount - 1 where ID = '005';[/code]

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