SQL：3个表，包含2个表中的选择列和一列上的总和

Question

TBL1

fld1 fld12 fld3 fld4

Tbl2

fld5 fld6 fld7 

TBL3

fld8 fld9 fld10 fld11 fld12 fld13

查询：

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。使用fld3的查询可以有效但sum(fld3)无效。请帮忙。

Answer 1

您必须拥有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sum功能如何运作？

Answer 2

某些数据库引擎要求您在使用聚合（sum，count avg etc）

时指定一个组

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

或以不同的方式写作

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

现在假设你在所有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