Like UNPIVOT in SQL! How to change the way that cols orders it self?

If we have a table like this

a        b        c
1        xx       xy
2        xy       yx
3        yy       xx


Since both of b and c are same type I would like to aggregate them in a one col call E respectively  

a        E
1        xx
1        xy
2        xy
2        yx
3        yy
3        xx


Is that possible using MYSQL 5.7 ?
You can just do it with a UNION ALL (you may or may not need the ORDER BY part, depending on your use-case):

SELECT
    a, b as `E`
FROM
    a
UNION ALL 
SELECT
    a, c as `E`
FROM
    a 
ORDER BY
    a ;


You can check it here.