sql - SQL：3个表，从2个表中选择列并在一列上求和

Question

表1

fld1 fld12 fld3 fld4

表2

fld5 fld6 fld7 

表3

fld8 fld9 fld10 fld11 fld12 fld13

询问：

select fld8, fld11, fld12, sum(fld3), fld4
from tbl1, tbl2, tbl3
where fld1=fld5 and fld2=fld6 and fld7=fld8

查询未运行。我使用了连接，但总和不起作用。查询fld3将起作用但不是sum(fld3)。请帮忙。

Answer 1

你必须有group by

select fld8, fld11, fld12, sum(fld3), fld4 
from tbl1, tbl2, tbl3 where fld1=fld5 and fld2=fld6 and fld7=fld8 
group by fld8, fld11, fld12, fld4

否则该sum功能将如何工作？

Answer 2

一些数据库引擎要求您在使用聚合时指定一个组（sum、count avg 等）

select fld8, fld11, fld12, sum(fld3), fld4
from tbl1, tbl2, tbl3
where fld1=fld5 and fld2=fld6 and fld7=fld8
GROUP BY fld8, fld11, fld12, fld4

或以不同的方式编写

select fld8, fld11, fld12, sum(coalesce(fld3,0)), fld4
from tbl1 
INNER JOIN tbl2 on fld1=fld5 and fld2=fld6
INNER JOIN tbl3 on  fld7=fld8
GROUP BY fld8, fld11, fld12, fld4

现在都假设您在所有 3 个表中都有记录。如果不是这种情况，您可能需要使用外连接并根据引擎在 sum 字段或 nvl 上合并。

所以也许...

SELECT fld8, fld11, fld12, sum(coalesce(fld3,0)), fld4
FROM  tbl1 
LEFT JOIN tbl2 on fld1=fld5 and fld2=fld6
LEFT JOIN tbl3 on  fld7=fld8
GROUP BY fld8, fld11, fld12, fld4