SQL switching values on a column
Odd question with terms like temporary storage. General SQL is denormalized so fetching when need rather than a transform.

Question 1: results and be an expression like:

SELECT id, sex, IF(sex='m', 'f', 'm') as antisex FROM table


Question 2: A variable can contain only a single value. So from ref manual on SELECT INTO:

SELECT id, sex, IF(sex='m', 'f', 'm') INTO @id, @sex, @antisex FROM table LIMIT 1