Question

I am facing an issue with a conditional join.

This is my query :

SELECT table_b.name
FROM table_a
LEFT JOIN table_b
ON table_a.id = table_b.a_id

The description of my table_b :

id, a_id, ref_z, name
----------------------
1 | 0   |   1  | James
2 | 0   |   2  | John
3 | 2   |   2  | John. S
4 | 0   |   3  | Martin
5 | 6   |   3  | Martin. M
6 | 2   |   3  | Martin. M. Mr

Let’s say I have a parameter @a_id.

I would like to join table_a and table_b ON table_a.id = @a_id if it exists else on 0.

So the output of the previous query will be (with @a_id = 2) :

James
John. S
Martin. M. Mr

Anyone has an idea, a blog post, a man page or anything else that can lead me to a correct query ?

Thank you,

Answer 1

where a_id = 2 

your join can be shorten into using (a_id) since both columns have the same name and there is no ambiguity.

on / using should not be confused for your predicate clause (where)