如图：有限制条件的分类求和语句怎么写？

[jshecn]

如图：如想找出2008-10-17前，direction为+减去direction为—的QTY总共是多少。怎么写？

PART_NO DIRECTION DATE QTY
100100001 + 2007-5-27 64
100100001 - 2007-9-25 3032
100100001 - 2008-6-30 231
100100001 - 2008-9-24 402
100100001 + 2008-8-14 600
100100001 - 2007-5-27 32
100100001 + 2008-6-30 7
100100001 + 2007-9-25 3000
100100001 - 2008-8-19 600
100100001 + 2008-9-10 402
100100001 + 2008-6-29 14
100100001 - 2008-10-17 2434
100100001 + 2008-5-26 224
100100001 - 2008-6-29 14
100100001 + 2008-10-17 2434

[Andy__Huang]

select part_no,direction,date,
sum(case when direction='+' then qty else -qty end) over(partition by part_no)
from tb
where to_char(date,'yyyy-mm-dd')<='2008-10-17';
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