Question

以下是我的查询示例：

SELECT ...
TRIM(eml.fxeml1) as "Email Address"
FROM pfship

LEFT JOIN mstfxem fax
ON fax.fxco=pfship.cshco AND fax.fxdiv=pfship.cshdiv AND fax.fxsold=pfship.cshsld
AND fax.fxtype='C' AND TRIM(fax.fxfax1) <> '' AND fax.fxdept LIKE '*F%'

LEFT JOIN mstfxem eml
ON eml.fxco=pfship.cshco AND eml.fxdiv=pfship.cshdiv AND eml.fxsold=pfship.cshsld
AND eml.fxtype='C' AND TRIM(eml.fxeml1) <> '' AND eml.fxdept LIKE '*E%'

WHERE ((pfship.cshco || '/' || pfship.cshdiv) = ?)
  AND (? = '*ALL' OR CAST(cshsld AS CHAR(15)) = ?) 
  AND ...
  ORDER BY TRIM(cshnme)

此查询应返回9条记录。当我删除：

LEFT JOIN mstfxem fax
ON fax.fxco=pfship.cshco AND fax.fxdiv=pfship.cshdiv AND fax.fxsold=pfship.cshsld
AND fax.fxtype='C' AND TRIM(fax.fxfax1) <> '' AND fax.fxdept LIKE '*F%'

LEFT JOIN mstfxem eml
ON eml.fxco=pfship.cshco AND eml.fxdiv=pfship.cshdiv AND eml.fxsold=pfship.cshsld
AND eml.fxtype='C' AND TRIM(eml.fxeml1) <> '' AND eml.fxdept LIKE '*E%'

我得到了9条记录，但有了它我得到了360条记录。我的加入有什么问题？

由于

我认为问题是1个人可以收到很多电子邮件或传真，我如何将结果分组为每个记录1个字符串，所以我最终只有9个记录？

Answer 1

您的加入没有任何意义，表格之间存在一对多的关系。

根据您的需要，有几种技术。首先是派生表。编写一个查询，从一对多表中只选择一条记录（您的要求应指定要选择的记录）。加入到那个。或者您可以在左连接上添加更多条件以仅获得一条记录。这些技术的一些例子：

select t1.field1, t2.email
from table1 t1 
join (select myid, max(email) as email from table2 group by myid) t2 
  on t1.myid = t2.myid

或

select t1.field1, t2.email
from table1 t1 
Left join from table2  t2 
  on t1.myid = t2.myid and t2.active = 1

或者，如果您希望所有电子邮件都位于逗号分隔列表中，则该代码是特定于数据库的，您需要告诉我们您使用的是哪个数据库后端。

获得大量重复？左加入不正确？

1 个答案: