Col_1         Col_2
====================
A
B              A
C              B
D              A
E              D
F              B

Col_1       Col_2
=================
A            B
A            D
B            C
B            F
D            E

SELECT
  Col_2 as Col_1,
  Col_1 as Col_2
FROM table
WHERE Col_2 IS NOT NULL
ORDER BY Col_2, Col_1