SQL 根据连接是否存在，从 table 有条件地查询 select 行

Question

我有一个连接 3 table 的查询，我正在尝试根据记录是否存在于第三个 table.

中检索一组条件行

如果第 3 个 table 中有匹配项，那么我只需要第一个 table 中的匹配记录。如果第 3 个 table 没有匹配项，那么我只想要第一个 table.

的一个记录

select
o.Object_ID,
reqCon.Connector_ID,
req.Object_ID as Requirement_ID

from t_object o

left join t_connector reqCon on reqCon.End_Object_ID = o.Object_ID and reqCon.Stereotype = 'deriveReqt'
left join t_object req on reqCon.Start_Object_ID = req.Object_ID and req.Stereotype = 'functionalRequirement'

这会产生以下类型的结果，但以粗体突出显示的是实际需要的行。

Object_ID   Connector_ID    Requirement_ID
40936   43259
40936   43260
**40936 43299   38013**
40943   43264
40943   43265
**40943 43298   38014**
**44088 46245**
44088   46246   
**42669 44655**
42669   44656
**42670 44657**
42670   44658

Answer 1

一个解决方案是union all：

select o.Object_ID, reqCon.Connector_ID, req.Object_ID as Requirement_ID
from t_object o join
     t_connector reqCon
     on reqCon.End_Object_ID = o.Object_ID and
        reqCon.Stereotype = 'deriveReqt' join
     t_object req
     on reqCon.Start_Object_ID = req.Object_ID and
        req.Stereotype = 'functionalRequirement'
union all
select o.Object_ID, min(reqCon.Connector_ID), null as Requirement_ID
from t_object o left join
     t_connector reqCon
     on reqCon.End_Object_ID = o.Object_ID and
        reqCon.Stereotype = 'deriveReqt' left join
     t_object req
     on reqCon.Start_Object_ID = req.Object_ID and
        req.Stereotype = 'functionalRequirement'
group by o.Object_Id
having sum(req.Object_Id is not null) = 0;

第一个查询引入了匹配项。第二个为没有匹配项的每个对象引入一行。

SQL 根据连接是否存在，从 table 有条件地查询 select 行

SQL Query to conditionally select a row from a table depending on join existence

mysql

sql

enterprise-architect