SQL-如何在JOIN条件下执行IF / ELSE

Question

I know how to perform an IF/ELSE in the join conditions, but is there a way I can use a different evaluation of the field altogether based on the ELSE?

SELECT o.id, s.id
FROM orders o
RIGHT JOIN subjects s ON s.name = 'Blah'
                     AND s.group_id = IF(o.service = 'Law Services', 2, ????)

In the above, I don't know what to put at ????

What I want to say is (pseudo code):

IF o.service = 'Law Services'

    JOIN subjects s ON s.name = 'Blah' AND s.group_id = 2

ELSE

    JOIN subjects s ON s.name = 'Blah'

Basically I only want to add the JOIN condition if s.group_id = 2 .

Answer 1

You can write the condition without an if :

SELECT o.id, s.id
FROM orders o RIGHT JOIN
     subjects s
     ON s.name = 'Blah' AND
        (o.service <> 'Law Services' or s.group_id = 2);

Answer 2

This may be worth a try:

SELECT o.id, s.id
FROM orders o
RIGHT JOIN subjects s 
   ON s.name = 'Blah'
     AND s.group_id = (CASE WHEN o.service = 'Law Services' THEN 2 ELSE s.group_id END)

Answer 3

SELECT o.id, s.id
FROM orders o
RIGHT JOIN subjects s ON s.name = 'Blah'
                     AND s.group_id = IF(o.service = 'Law Services', 2, s.group_id)