Different JOIN values depending on the value of another column - sql

Question

I have 2 tables j and c.
Both tables have columns port and sec.
For j.port = ABC, I want to join the 1st 6 characters of c.sec with the 1st 6 characters of j.sec.
For other j.ports, I want to join c.sec = j.sec
How can I do that ?
select c.port,j.port,c.sec,j.sec from j, c
where c.SEC =
CASE WHEN j.port = 'ABC' then SUBSTRING(c.sec,1,6) = SUBSTRING(j.sec,1,6)
--> something like this
else j.sec

Answer 1

Performance wise breaking this into two may be beneficial. The complex join condition will force nested loops otherwise.
SELECT c.port,
j.port,
c.sec,
j.sec
FROM j
JOIN c
ON LEFT(c.sec, 6) = LEFT(j.sec, 6)
WHERE j.port = 'ABC'
UNION ALL
SELECT c.port,
j.port,
c.sec,
j.sec
FROM j
JOIN c
ON c.sec = j.sec
WHERE j.port IS NULL
OR j.port <> 'ABC'
Or in this specific case you could also do
SELECT c.port,
j.port,
c.sec,
j.sec
FROM j
JOIN c
ON LEFT(c.sec, 6) = LEFT(j.sec, 6)
and (j.port = 'ABC' OR c.sec = j.sec)
This allows the main join to be a simple equi join that can use any of the join algorithms with a residual predicate on the result.
For the following example data both of these took about 700ms on my machine whereas I killed the three competing answers after 30 seconds each as none of them completed in that time.
create table c(port varchar(10), sec varchar(10) index ix clustered )
create table j(port varchar(10), sec varchar(10))
INSERT INTO c
SELECT TOP 1000000 LEFT(NEWID(),10) , LEFT(NEWID(),10)
FROM sys.all_objects o1, sys.all_objects o2
INSERT INTO j
SELECT TOP 1000000 LEFT(NEWID(),10) , LEFT(NEWID(),10)
FROM sys.all_objects o1, sys.all_objects o2

Answer 2

You could use:
select c.port,j.port,c.sec,j.sec
from j
join c
on (CASE WHEN j.port = 'ABC' and SUBSTRING(c.sec,1,6) = SUBSTRING(j.sec,1,6) then 1
WHEN c.sec = j.sec THEN 1
END) = 1
The same as:
select c.port,j.port,c.sec,j.sec
from j
join c
on (j.port = 'ABC' and SUBSTRING(c.sec,1,6) = SUBSTRING(j.sec,1,6))
or (c.SEC = j.sec AND (j.port <> 'ABC' or j.port IS NULL))