Oracle 11g Release 2

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CREATE TABLE tab_1
AS
SELECT 'A' col_2
       FROM dual
UNION ALL
SELECT 'B' col_2
       FROM dual
UNION ALL
SELECT 'C' col_2
       FROM dual
UNION ALL
SELECT 'N' col_2
       FROM dual;
4 rows affected

CREATE TABLE tab_2
AS
SELECT 'CCNCCN' col_2
       FROM dual
UNION ALL
SELECT 'ABCNSN' col_2
       FROM dual
UNION ALL       
SELECT 'TABGNN' col_2
       FROM dual;
3 rows affected

WITH
cte (c, r)
AS
(
SELECT substr(t2.col_2, 1, 1) c,
       substr(t2.col_2, 2, length(t2.col_2) - 1) r
       FROM tab_2 t2
UNION ALL
SELECT substr(c.r, 1, 1) c,
       substr(c.r, 2, length(c.r) - 1) r
       FROM cte c
       WHERE substr(c.r, 1, 1) IS NOT NULL
)
SELECT DISTINCT
       c.c
       FROM cte c
       WHERE NOT EXISTS (SELECT *
                                FROM tab_1 t1
                                WHERE t1.col_2 = c.c);


C


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