sql - Conditionally fallback to different join condition if stricter condition not matched

Question

I have 2 tables j and c.

Both tables have columns ports and sec, and JOIN ON j.ports = c.ports and c.sec = j.sec.

For j.port = 'ABC', if there is no c.sec = j.sec for the same ports, then JOIN ON LEFT(c.sec, 6) = LEFT(j.sec, 6)

For other j.ports, I only want to join ON j.ports = c.ports and c.sec = j.sec

How can I do that?

Example Data

Table c

+------+------------+------------+
| Port |    sec     |   Other    |
+------+------------+------------+
| ABC  | abcdefghij |  ONE       |
| ABC  | klmnop     |  TWO       |
| LMN  | qwertyuiop |  THREE     |
| XYZ  | asdfghjkl  |  FOUR      |
+------+------------+------------+

Table j

+------+------------+
| Port |    sec     |
+------+------------+
| ABC  | abcdefxxxx |
| ABC  | klmnop     |
| LMN  | qwertyuiop |
| XYZ  | zxcvbnm    |
+------+------------+

EDITED: Desired Results

+------+------------+------------+
| Port |    sec     |  other     |
+------+------------+------------+
| ABC  | abcdefghij |  ONE       |  --> mactching on sec's 1st 6 characters 
| ABC  | klmnop     |  TWO       |  --> mactching on sec
| LMN  | qwertyuiop |  THREE     |  --> mactching on sec
+------+------------+------------+

Answer 1

This does conditional joining:

select t1.*, t2.*
from j t1 inner join c t2
on t2.ports = t1.ports and
  case 
    when exists (select 1 from c where sec = t1.sec) then t1.sec 
    else left(t1.sec, 6) 
  end =
  case 
    when exists (select 1 from c where sec = t1.sec) then t2.sec 
    else left(t2.sec, 6) 
  end

I question its efficiency but I think it does what you need.
See the demo.