Возможно, оффтоп, но ветки по "чистому" SQL не нашел.

Есть три таблицы:
erwin.Subject_Plan(StructurePlanAID[FK]. SubjectAID[FK],CIRCLEID,SubCircleID, data)
[[PK]==(StructurePlanAID[FK]. SubjectAID[FK])]
erwin.Structure_Plan(StructurePlanAID[pK],TitlePlanAID)
erwin.subject(SubjectAID[PK],SubjectName)

В чем разница в этих двух запросах. Первый выдает нужные данные. Второй - полную чушь.

1.
select * from erwin.Subject_Plan AS SuP
left join
erwin.Structure_Plan AS StP
on (SuP.StructurePlanAID = StP.StructurePlanAID)
left join erwin.subject AS S
on (S.SubjectAID=SuP.SubjectAID)
where TitlePlanAID=1
and CIRCLEID='ГСЭ.00'
and SubCircleID='ГСЭ.В.00'
and SubCircleID IS NOT NULL
;

2.
select * from erwin.Subject_Plan AS SuP
left join
( select * from
erwin.Structure_Plan
where TitlePlanAID=1
and CIRCLEID='ГСЭ.00'
and SubCircleID='ГСЭ.В.00'
and SubCircleID IS NOT NULL
)AS StP
on (SuP.StructurePlanAID = StP.StructurePlanAID)
left join erwin.subject AS S
on (S.SubjectAID=SuP.SubjectAID)