It's happen when I import moodle multiple question XML format , but the XML didn't include the answer (my mistake when creating XML format). The question imported with no answer option. The question can not deleted. So I did delete with SQL.
delete from mdl_question where id IN
(
SELECT
aa.id
FROM
(select DISTINCT id from mdl_question) AS aa
LEFT JOIN mdl_qtype_multichoice_options bb ON aa.id = bb.questionid
WHERE
bb.questionid IS NULL
)