Float vs Decimal in MySQL data types

[s0c0]

I have this query:

 

SELECT (sum(wsale_cost)*sum(quantity)) as cost FROM wsale_orderedProd WHERE invoice='14277333'

 

The wsale_cost datatype is float(5,2) and that query yeilds the following result: 170.50000190735 on a calculator it yields 170.50.  I realized that I can wrap ROUND(x,2) where x is the formula and get it look like 170.50, but why the hell does float do this? 

 

Should I instead be using decimal?  I've read that float has more efficient data storage.  Strange...can't find a good answer on google, maybe one of you can help.  For right now I'm switching to decimal(5,2) to avoid having to run an additional function inside my query.

[fenway]

You rarely want FLOAT... especially not for anything having to do with currency.

[aschk]

If you want to know why it's doing this read this : http://docs.sun.com/source/806-3568/ncg_goldberg.html