WITH TALLY
     AS (
     SELECT 1 N
     UNION ALL
     SELECT 2
     UNION ALL
     SELECT 3
     UNION ALL
     SELECT 4
     UNION ALL
     SELECT 5
     UNION ALL
     SELECT 6
     UNION ALL
     SELECT 7
     UNION ALL
     SELECT 8
     UNION ALL
     SELECT 9
     UNION ALL
     SELECT 10
     UNION ALL
     SELECT 11
     UNION ALL
     SELECT 12
     UNION ALL
     SELECT 13
     UNION ALL
     SELECT 14
     UNION ALL
     SELECT 15
     UNION ALL
     SELECT 16
     UNION ALL
     SELECT 17
     UNION ALL
     SELECT 18
     UNION ALL
     SELECT 19
     UNION ALL
     SELECT 20
     UNION ALL
     SELECT 21
     UNION ALL
     SELECT 22
     UNION ALL
     SELECT 23
     UNION ALL
     SELECT 24
     UNION ALL
     SELECT 25
     UNION ALL
     SELECT 26),
     DEUX
     AS (SELECT T1.N C1,
                T2.N C2
         FROM TALLY T1
              CROSS JOIN TALLY T2),
     TROIS
     AS (SELECT T1.C1,
                T1.C2 C2,
                T2.N C3
         FROM DEUX T1
              CROSS JOIN TALLY T2),
     QUATRE
     AS (SELECT T1.C1,
                T1.C2 C2,
                T1.C3 C3,
                T2.N C4
         FROM TROIS T1
              CROSS JOIN TALLY T2),
     T
     AS (
     SELECT CHAR(64+N) ALPHA
     FROM TALLY
     UNION ALL
     SELECT CHAR(64+C2) + CHAR(64+C1)
     FROM DEUX
     UNION ALL
     SELECT CHAR(64+C3) + CHAR(64+C2) + CHAR(64+C1)
     FROM TROIS
     UNION ALL
     SELECT CHAR(64+C4) + CHAR(64+C3) + CHAR(64+C2) + CHAR(64+C1)
     FROM QUATRE)
     SELECT *
     FROM T AS T1