declare @a table (aID int, userID int, account varchar(100))
insert @a values (31,41,'Клиент 1'), (32,42,'Клиент 2'), (33,43,'Клиент 3')

declare @sc table (scID int, aID int, naim varchar(100))
insert @sc values (11,31,'Договор 1'), (12,32,'Договор 2'), (13,33,'Договор 3')

declare @proj table (scID int, projName varchar(100))
insert @proj values (11,'Проект 1'), (12,'Проект 2'), (14,'Проект 4')

-- Не выдаст "Договор 3", хотя связь идет через left join и aID=33 есть в обеих таблицах
select a.account, sc.naim, p.projName
from @a a
left join @sc sc on a.aID = sc.aID
inner join @proj p on sc.scID = p.scID

-- Решение 1. Через скобки
select a.account, sc.naim, p.projName
from @a a
left join (@sc sc inner join @proj p on sc.scID = p.scID)
on a.aID = sc.aID

-- Решение 2. Через подзапрос
;with tmp as (
  select sc.aID, sc.naim, p.projName
  from @sc sc inner join @proj p on sc.scID = p.scID
) 
select a.account, tmp.naim, tmp.ProjName
from @a a
left join tmp on a.aID = tmp.aID

if object_id(N'integerNumbers','u') is not null drop table integerNumbers
go
Create table integerNumbers (num int)

insert integerNumbers
select n6.num*100000 + n5.num*10000 + n4.num*1000 + n3.num*100 + n2.num*10 + n1.num [num] -- Миллион записей от 0 до 999999
from
(select 0 as num union all select 1 union all select 2 union all select 3 union all select 4 union all select 5 union all select 6 union all select 7
 union all select 8 union all select 9) n1,
(select 0 as num union all select 1 union all select 2 union all select 3 union all select 4 union all select 5 union all select 6 union all select 7
 union all select 8 union all select 9) n2,
(select 0 as num union all select 1 union all select 2 union all select 3 union all select 4 union all select 5 union all select 6 union all select 7
 union all select 8 union all select 9) n3,
(select 0 as num union all select 1 union all select 2 union all select 3 union all select 4 union all select 5 union all select 6 union all select 7
 union all select 8 union all select 9) n4,
(select 0 as num union all select 1 union all select 2 union all select 3 union all select 4 union all select 5 union all select 6 union all select 7
 union all select 8 union all select 9) n5,
(select 0 as num union all select 1 union all select 2 union all select 3 union all select 4 union all select 5 union all select 6 union all select 7
 union all select 8 union all select 9) n6
order by 1

go
create clustered index IX_Num on integerNumbers (Num)
go

select * from integerNumbers
where num between 1 and 15