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select
sum(case a when 0 then b else 0 end) as qty1,
sum(case a when 1 then b else 0 end) as qty2,
sum(b) - qty1 - qty2 as qty3  //qty3ÎÒÊÇÏëʵÏÖÕâÑùµÄ¹¦ÄÜ£¬µ«ÎÒÖªµÀÕâÑùд¿Ï¶¨²»¶Ô£¬²»ÖªµÀÔõôÄÜʵÏÖ£¿
from table1
group by...

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SQL code:
select
sum(case a when 0 then b else 0 end) as qty1,
sum(case a when 1 then b else 0 end) as qty2,
sum(b) - sum(case a when 0 then b else 0 end)-sum(case a when 1 then b else 0 end) as qty3
from table1
group by...

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sum(b) - qty1 - qty2 as qty3
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sum(case a when 0 then 0 when 1 then 0 else b end) as qty3

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sum(b) - qty1 - qty2 as qty3 Ìæ»»³Ésum(b)-sum(case when a=0 or a=1 then b else 0)

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select qty1,qty2,b-qty1-qty2
from(
select
sum(case a when 0 then b else 0 end) as qty1,
sum(case a when 1 then b else 0 end) as qty2,
sum(b) as b
from table1
group by..) b

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sum(b) - qty1 - qty2 as qty3
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sum(case a when 0 then 0 when 1 then 0 else b end) as qty3

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select
sum(case a when 0 then b else 0 end) as qty1,
sum(case a when 1 then b else 0 end) as qty2,
sum(b) - qty1 - qty2 as qty3  //qty3ÎÒÊÇÏëʵÏÖÕâÑùµÄ¹¦ÄÜ£¬µ«ÎÒÖªµÀÕâÑùд¿Ï¶¨