解法1

UPDATE salary
SET
    sex = CASE sex
        WHEN 'm' THEN 'f'
        ELSE 'm'
    END;

解法2

update salary set sex = char(ascii('m') + ascii('f') - ascii(sex));

解法3

update salary
set sex = replace("fm", sex, "")

解法4

update salary set sex = char(ASCII(sex) ^ ASCII('m') ^ ASCII('f') ) ;