这个SQL语句怎么写？

1，表class中的字段c_id与stu表中的s_cid为关联字段
2，返回字段 c_name,s_no,s_name
3,查询条件：c_stu字段值大于20，并且s_no第二到四位是“006”这三个字符
4，查询结果按s_no倒排序
表中字段名不重复，字段名前不用加表名

这样写怎么错了？
select c_name,s_no,s_name from class,stu where c_id = s_cid and c_stu>20

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select c_name,s_no,s_name from class,stu where c_id = s_cid and c_stu>20 and s_no like '?006*' order by s_no desc
错哪里了？

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select c_name,s_no,s_name from class,stu where c_id=s_cid and c_stu>20 and s_no like '?006*' order by s_no desc

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select [class].c_name,stu.s_no,stu.s_name from [class],stu where[class].c_id = stu.s_cid and stu.c_stu>20

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select [class].c_name,[stu].s_no,[stu].s_name from [class],[stu] where [class].c_id=[stu].s_cid and [class].c_stu>20 and [sut].s_no like '_006%' order by [stu].s_no desc