下面这端，用于取2个表的插值 选择表单里有NULL值的表。
SELECT * FROM `MESSAGE_TEXT` AS A LEFT JOIN `MESSAGE` AS B ON A.id = B.messageid WHERE A.recid = 27 AND B.id IS NULL

但是写成TP后，显示 array(0) { }
$Message = M()
->table(MESSAGE_TEXT.' A')
->join (MESSAGE." B ON A.id = B.messageid")
->field('A.id,A.createby,A.recid')
->where(array('A.recid'=>$uid,'B.id'=>array('exp','is NULL')))
->select();