12
SELECT a,b,a+b
FROM test_table
INTO OUTFILE '/tmp/result.txt'
FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"'
LINES TERMINATED BY '\n'
(the docs show INTO OUTFILE up in the SELECT .. portion which may work as well, but I've never tried it that way) http://dev.mysql.com/doc/refman/5.0/en/select.html
INTO OUTFILE creates a file on the server; if you are on a client and want it there, do:
mysql -u you -p -e "SELECT ..." > file_name