SQL conditional join with condition priorities like in CASE statement - sql

Question

How to join conditionally on (1) full key OR (2) part of the key, if first condition is not met. Suppose id consists of 7 characters.
select
a.id,
coalesce (t.somevalue, t2.somevalue)
from a
left join t
on a.id=t.id
left join t as t2
on left(a.id,6)=left(t2.id,6)
Is the above join possible in one shot? Something like this but maintaining priority of full key join over part key join?
left join t
on
a.id=t.id OR left(a.id,6)=left(t2.id,6)
I wonder if there is something like
left join t
on
case when condition 1
case when condition 2
Here is sample data and expected results.
+---------+
| a.id |
+---------+
| aaaaa0A |
| aaaaa0B |
| aaaaa1A |
| aaaaa2A |
| aaaaa3B |
+---------+
+---------+
| t.id |
+---------+
| aaaaa0C |
| aaaaa1B |
| aaaaa1A |
| aaaaa2B |
| aaaaa3A |
+---------+
+---------+---------+--------------+
| a.id | t.id | condition no |
+---------+---------+--------------+
| aaaaa0A | aaaaa0C | 2 |
| aaaaa0B | aaaaa0C | 2 |
| aaaaa1A | aaaaa1A | 1 |
| aaaaa2A | aaaaa2B | 2 |
| aaaaa3B | aaaaa3A | 2 |
+---------+---------+--------------+

Answer 1

I think you want outer apply:
select a.id, t.somevalue
from a outer apply
(select top (1) t.*
from t
where left(a.id, 6) = left(t.id, 6)
order by (case when a.id = t.id then 1 else 2 end) -- put full matches first
) t;

Answer 2

You can do a 2nd LEFT join of t under the condition that the 1st join does not match:
select
a.id,
coalesce(t1.id, t2.id) id,
case
when t1.id is not null then 1
when t2.id is not null then 2
else 0
end [condition no]
from a
left join t as t1 on a.id = t1.id
left join t as t2 on t1.id is null and left(a.id, 6) = left(t2.id, 6)
See the demo.
Results:
> id | id | condition no
> :------ | :------ | -----------:
> aaaaa0A | aaaaa0C | 2
> aaaaa0B | aaaaa0C | 2
> aaaaa1A | aaaaa1A | 1
> aaaaa2A | aaaaa2B | 2
> aaaaa3B | aaaaa3A | 2