Well, this is ugly, but seems to do the trick:
WITH data AS (
SELECT '123' AS val FROM dual
UNION SELECT '12' FROM dual
UNION SELECT '1' FROM dual
UNION SELECT '12345' FROM dual
UNION SELECT '9612' FROM dual -- ok
UNION SELECT '9613' FROM dual -- wrong
UNION SELECT '1296' FROM dual -- wrong
UNION SELECT 'AAAA' FROM dual -- wrong
)
SELECT
CASE
WHEN length(val) != 4
OR NOT regexp_like(val, '[0-9]{4}')
OR NOT to_number(substr(val, 3, 2)) BETWEEN 1 AND 12
THEN to_date('0010', 'YYMM')
ELSE to_date(val, 'YYMM')
END AS date_yymm
FROM data;
Output:
DATE_YYMM ---------- 01-10-2000 01-10-2000 01-10-2000 01-10-2000 01-10-2000 01-12-2096 01-10-2000 01-10-2000
Or you can just use a function:
CREATE OR REPLACE FUNCTION is_date_format(p_value IN VARCHAR2, p_format IN VARCHAR2) RETURN NUMBER
AS
l_dummy DATE;
BEGIN
l_dummy := to_date(p_value, p_format);
RETURN 0;
EXCEPTION
WHEN OTHERS THEN
RETURN 1;
END;
/
WITH data AS (
SELECT '123' AS val FROM dual
UNION SELECT '12' FROM dual
UNION SELECT '1' FROM dual
UNION SELECT '12345' FROM dual
UNION SELECT '9612' FROM dual -- ok
UNION SELECT '9613' FROM dual -- wrong
UNION SELECT '1296' FROM dual -- wrong
)
SELECT
-- fx for exact matching as pointed out by Nicholas Krasnov
DECODE(is_date_format(val, 'fxYYMM'), 0, val, '0010')
FROM data;