I need to retrieve data filtered by the results of a calculated column - how?

INFORMIX:

SELECT

TUD_PK_LIST_NO AS PKL,
TUP_TAG_NO AS TAG,
(
CASE
(
CASE (SELECT ttt_prnt_tag_no FROM INTTTT_REC WHERE ttt_itm_ctl_no = tup_itm_ctl_no) WHEN NULL THEN (SELECT MIN(pkc_cmpt_tag_no) FROM INTPKC_REC WHERE pkc_pkg_ctl_no = tup_itm_ctl_no GROUP BY pkc_pkg_ctl_no) ELSE (SELECT ttt_prnt_tag_no FROM INTTTT_REC WHERE ttt_itm_ctl_no = tup_itm_ctl_no) END )
WHEN NULL
THEN (SELECT TRIM(tag_mst_tag_no) FROM INTTAG_REC WHERE tag_itm_ctl_no = tup_itm_ctl_no)
ELSE
(
CASE
(
SELECT tag_mst_tag_no
FROM
INTTAG_REC
WHERE tag_itm_ctl_no =
(
CASE (SELECT ttt_itm_ctl_no FROM INTTTT_REC WHERE ttt_itm_ctl_no = tup_itm_ctl_no)
WHEN NULL
THEN (SELECT MIN(pkc_cmpt_ctl_no) FROM INTPKC_REC WHERE pkc_pkg_ctl_no = tup_itm_ctl_no GROUP BY pkc_pkg_ctl_no)
ELSE (SELECT ttt_itm_ctl_no FROM INTTTT_REC WHERE ttt_itm_ctl_no = tup_itm_ctl_no)
END
)

)
WHEN NULL
THEN (SELECT TRIM(atg_mst_tag_no) FROM INAATG_REC WHERE atg_itm_ctl_no =
(
CASE (SELECT ttt_itm_ctl_no FROM INTTTT_REC WHERE ttt_itm_ctl_no = tup_itm_ctl_no)
WHEN NULL
THEN (SELECT MIN(pkc_cmpt_ctl_no) FROM INTPKC_REC WHERE pkc_pkg_ctl_no = tup_itm_ctl_no GROUP BY pkc_pkg_ctl_no)
ELSE (SELECT ttt_itm_ctl_no FROM INTTTT_REC WHERE ttt_itm_ctl_no = tup_itm_ctl_no)
END
)
)
ELSE
(
SELECT TRIM(tag_mst_tag_no)
FROM
INTTAG_REC
WHERE tag_itm_ctl_no =
(
CASE (SELECT ttt_itm_ctl_no FROM INTTTT_REC WHERE ttt_itm_ctl_no = tup_itm_ctl_no)
WHEN NULL
THEN (SELECT MIN(pkc_cmpt_ctl_no) FROM INTPKC_REC WHERE pkc_pkg_ctl_no = tup_itm_ctl_no GROUP BY pkc_pkg_ctl_no)
ELSE (SELECT ttt_itm_ctl_no FROM INTTTT_REC WHERE ttt_itm_ctl_no = tup_itm_ctl_no)
END
)
)
END
)
END
) AS MASTER,


FROM BLA BLA BLA
WHERE `MASTER` = '123456'



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