请问您的Database是哪一种？如果是Oracle，可以试试
from A as a order by a.b asc null last
如果是MySQL，可以试试用减号：
from A as a order by -a.b desc
让B字段降序排列就可以了order by b desc
用联合查询试试SELECT * FROM A WHERE b IS NOT NULL ORDER BY b UNION SELECT * FROM A WHERE b IS NULL
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