Handling Mysql "UNKNOWN" data type

MySQL source table:
CREATE TABLE hnt_noti_confirm(
user_id varchar(10),
confirm_memo_idx int NOT NULL primary key,
is_open char(1),
opendate date,
regdate date);

Add SQL table as following:
SELECT
a.user_id AS memberid,
a.confirm_memo_idx AS notimastseq,
CASE WHEN is_open='Y' THEN a.opendate
WHEN is_open='N' THEN a.regdate END AS confirmdt,
'R' AS status
FROM
hnt_noti_confirm a