Don't use DOUBLE (or FLOAT) for monetary values. Use DECIMAL(...). Two
decimal places ( ,2) suffices for most countries, but not all.
DOUBLE values have been rounded when converted from decimal to binary.
Multiply creates another rounding. The end result may look the same when
converted
I suspect, without knowing for sure, that the server will preallocate the 16MB
(or whatever) and then proceed to fill it in. Such a malloc is not costly, and
not risky (unless you have thousands of threads doing the same thing at the
same time).
Who cares? If GROUP_CONCAT() is what you need,
