联接3表的MySQL，并得到错误

Question

有人可以告诉我下面的代码有什么问题吗？

SELECT 
u.regid AS `ID`,
SUBSTRING_INDEX(u.firstname,',','1') AS `First Name`, 
SUBSTRING_INDEX(u.lastname,',','1') AS `Last Name`,  
u.city1 AS City, 
u.state1 AS State, 
h.confirmation_number AS `Confirmation Number`,
u.hotel_rooming_list_id AS `Hotel Rooming List ID`, 
u.regcode AS RegCode,
COUNT(*) AS `Total Over 18`,
u.regnum AS `Total People`

FROM 
reg2018 AS u, 
mealroom2018 AS i

INNER JOIN hotel_confirmation_number AS h ON  h.regcode = u.regcode

WHERE u.cancel is NULL AND 
u.mealroom = 'Y'  AND 
i.regcode = u.regcode AND
i.year < '2000'

GROUP BY u.regid
ORDER BY u.regid ASC

我得到的错误是＃1054-'on子句'中的未知列'u.regcode'