你可以用 using :

delete from t
    using (select scanId, min(scandatetime) as min_sdt, max(scandatetime) as max_sdt
           from t
           group by scanid
          ) tt
    where tt.scanId = t.scanId and t.scandatetime not in (tt.min_sdt, tt.max_sdt);

你也可以这样说：

delete from t
    where scandatetime <> (select min(t2.scandatetime) from t tt where tt.scanid = t.scanid) and
          scandatetime <> (select max(t2.scandatetime) from t tt where tt.scanid = t.scanid) ;

可以将“不在”与子选择一起使用：

delete from the_table t1
where (scanid, scandatetime) not in (select scanid, min(scandatetime)
                                     from the_table
                                     group by scanid
                                     union all
                                     select scanid, max(scandatetime)
                                     from the_table
                                     group by scanid);

但我假设解决方法是 exists 会更快。

如果上面（至少）有一条记录，下面（至少）有一条记录，则记录位于中间：

DELETE FROM ztable d
WHERE EXISTS ( SELECT *         
        FROM  ztable x
        WHERE x.scanId = d.scanId
        AND x.scandatetime < d.scandatetime
        )
AND EXISTS ( SELECT *
        FROM  ztable x
        WHERE x.scanId = d.scanId
        AND x.scandatetime > d.scandatetime
        );

类似的技巧，使用 row_number() :

DELETE FROM ztable d
USING ( SELECT scanId, scandatetime
        , row_number() OVER 
                (PARTITION BY scanId ORDER BY scandatetime ASC) rn
        , row_number() OVER
                (PARTITION BY scanId ORDER BY scandatetime DESC) rrn
        FROM  ztable 
        ) x
        WHERE x.scanId = d.scanId
        AND x.scandatetime = d.scandatetime
        AND x.rn <> 1 AND x.rrn <> 1

        ;