SQL Inner join on one field when second is null and on second when first is null

Question

OP is completely new to databases and SQL, so this question may be answered somewhere else but I don't quite have the vocabulary to find what I'm looking for; a push in the right direction would be great.

I'm trying to make a view of two tables inner joined. This is currently how they look:

and this is how I would like them to look:

The issue with this is that the view is empty because c4 and c5 can be null values.

I essentially want these two later inner joins on c4 and c5 to happen if one of them has a value.

Just to be thorough:

1. If c4 exists, inner join on it.
2. If c5 exists, inner join on it.
3. If neither exist, don't inner join on it.

Each of these are preceded by the inner joins between UTC and colNum. By this I mean that the UTC and colNum joins always happen.

I know that sql is a query language, so it does not do computation, but there must be a filter that will allow for this logic to be applied to these two tables.

It is useful to note that if c4 exists, c5 is null and if c5 exists, c4 is null, and that I still want a row (joined based on the previous two inner joins) if both are null.

Again, I don't really know the language surrounding SQL, so my effort to find an answer before asking one was hampered. If something like this has already been answered, just point me to it.

Answer 1

It is a little big statement to do it in comment so I will post it as an answer. If my understanding of the problem is correct then it will be like:

select * 
from sizeconditionstable t1
join specalloytable t2
on (t1.c4 is not null and t2.c4 is not null and t1.c4 = t2.c4) or 
   (t1.c5 is not null and t2.c5 is not null and t1.c5 = t2.c5)

Edit:

select * 
    from sizeconditionstable t1
    join specalloytable t2
    on (t1.utc = t2.utc and t1.colnum = t2.colnum) and
       ((t1.c4 = t2.c4) or (t1.c4 is null and t2.c4 is null)) and
       ((t1.c5 = t2.c5) or (t1.c5 is null and t2.c5 is null))

This is the version which will join always on utc and colnum and also on c4 and c5 if they are filled in both tables.