请教一个UNION and ORDER BY报错问题

SQL code

SELECT q.* 
FROM mdl_question q 
INNER JOIN mdl_question_categories qc ON 
    qc.id = q.category 
    AND q.hidden='0' 
    AND q.parent='0' 
LEFT JOIN mdl_question_attributes qa ON 
    q.id = qa.q_id 
WHERE qtype = 'match' 
    AND IFNULL( qa.commonmodulus, 0.1 ) >= 0.1 
    AND IFNULL( qa.commonmodulus, 0.1 ) <= 0.5 
    AND FIND_IN_SET(qc.contextid,'10, 3, 1') 
    ORDER BY RAND() ASC limit 1 
  [color=#FF0000]union all[/color] 
SELECT q.* 
FROM mdl_question q 
INNER JOIN mdl_question_categories qc ON 
    qc.id = q.category AND q.hidden='0' 
    AND q.parent='0' 
LEFT JOIN mdl_question_attributes qa ON 
    q.id = qa.q_id 
WHERE qtype = 'match' 
AND IFNULL( qa.commonmodulus, 0.1 ) >= 0.5 
AND IFNULL( qa.commonmodulus, 0.1 ) <= 0.9 
AND FIND_IN_SET(qc.contextid,'10, 3, 1') 
ORDER BY RAND() ASC limit 1

上面这段主要是想把两个用了ORDER BY RAND()的union all起来，
不过执行的时候报错，要是想通过的话，该怎么改呢？

第一个select和第二个select差不多，都是从抽出的数据里面随机抽取一条数据。

select * from (
SELECT q.* 
FROM mdl_question q 
INNER JOIN mdl_question_categories qc ON 
  qc.id = q.category 
  AND q.hidden='0' 
  AND q.parent='0' 
LEFT JOIN mdl_question_attributes qa ON 
  q.id = qa.q_id 
WHERE qtype = 'match' 
  AND IFNULL( qa.commonmodulus, 0.1 ) >= 0.1 
  AND IFNULL( qa.commonmodulus, 0.1 ) <= 0.5 
  AND FIND_IN_SET(qc.contextid,'10, 3, 1') 
  ORDER BY RAND() ASC limit 1 ) a
union all
select * from ( 
SELECT q.* 
FROM mdl_question q 
INNER JOIN mdl_question_categories qc ON 
  qc.id = q.category AND q.hidden='0' 
  AND q.parent='0' 
LEFT JOIN mdl_question_attributes qa ON 
  q.id = qa.q_id 
WHERE qtype = 'match' 
AND IFNULL( qa.commonmodulus, 0.1 ) >= 0.5 
AND IFNULL( qa.commonmodulus, 0.1 ) <= 0.9 
AND FIND_IN_SET(qc.contextid,'10, 3, 1') 
ORDER BY RAND() ASC limit 1) b