Store Sum to a memory variable

[Michael Smith]

I am attempting to store the Sum of Taxes to a memory variable with the code below:

SELECT SUM(TOTTAX) AS mSumTaxes FROM Invoices02 group by Invnbr;

But when I try to use the variable 'mSumTaxes' later in the code block, MySql does not accept it.

How do I do this?

Mike