SQL查询后，如何得到两条记录之间的区别？

Question

我想查询后得到两个记录之间的差异。

这是查询：

select count(t.customerid) as NumberofCustomers, st.regionid, v.vendorid
from region r join store st
on r.regionid = st.regionid
join salestransaction t 
on st.storeid = t.storeid
join soldvia s
on t.tid = s.tid
join product p
on s.productid = p.productid
join vendor v
on p.vendorid = v.vendorid
where v.vendorid = 2
group by st.regionid, v.vendorid

这是查询产生的结果：

NumberofCustomers   regionid    vendorid
     495               1            2
     329               2            2

如何获得第1行和第2行中的NumberofCustomers之间的区别？ 我尝试使用差异来实现此目的，但是并没有给我想要的结果。

任何帮助，将不胜感激。

Answer 1

你的目标很奇怪。 但是我为您提供了有趣而怪异的解决方案：

SELECT t.*,
   IF(@prev IS NULL, t.NumberofCustomers, (t.NumberofCustomers- @prev)) difference,
   @prev := t.NumberofCustomers
FROM )
select count(t.customerid) as NumberofCustomers, 
   st.regionid, v.vendorid
from region r join store st
on r.regionid = st.regionid
join salestransaction t 
on st.storeid = t.storeid
join soldvia s
on t.tid = s.tid
join product p
on s.productid = p.productid
join vendor v
on p.vendorid = v.vendorid
where v.vendorid = 2
group by st.regionid, v.vendorid ) t

Answer 2

如果只有两个区域，则可以使用条件聚合：

select v.vendorid,
       sum(st.regionid = 1) as NumRegion1,
       sum(st.regionid = 2) as NumRegion2,
       (sum(st.regionid = 1) - sum(st.regionid = 2)) as diff
from region r join
     store st
     on r.regionid = st.regionid join
     salestransaction t 
     on st.storeid = t.storeid join
     soldvia s
     on t.tid = s.tid join
     product p
     on s.productid = p.productid join
     vendor v
     on p.vendorid = v.vendorid
where v.vendorid = 2
group by v.vendorid;

返回的数字都是一行而不是两行。