原始数据如下：

drop table test_gen purge;

create table test_gen as
(
select 1 as seqno,1 as amount from dual
union all
select 2 as seqno,null as amount from dual
union all
select 3 as seqno,null as amount from dual
union all
select 4 as seqno,null as amount from dual
union all
select 5 as seqno,2 as amount from dual
union all
select 6 as seqno,null as amount from dual
union all
select 7 as seqno,null as amount from dual
union all
select 8 as seqno,3 as amount from dual
union all
select 9 as seqno,null as amount from dual
union all
select 10 as seqno,null as amount from dual
union all
select 11 as seqno,5 as amount from dual
);
复制

要求：希望通过SQL查出一个虚拟列AMOUNT_ALL，该列以SEQNO的顺序根据AMOUNT字段的值填充，如果AMOUNT有值则虚拟列等于该条记录中AMOUNT的值，如果AMOUNT的值为NULL，则该虚拟列为上一条AMOUNT不为NULL的值，输出示例如下：

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