SQL conditional JOIN - JOIN point defined based on joined_table conditions - mysql

Question

Scenario:
In each of the below, "created date", "other_created_date" and "date" are a Day (i.e. 2012-01-03)
Table 1:
fields:
ID | created_date
Table 2:
fields:
ID | table_1_fk | other_created_date
Table 3:
fields:
date
The Goal:
I want to do the following:
SELECT * FROM table_1
JOIN table_2
ON table_1.id = table_2.table_1_fk
FULL OUTER JOIN table_3
ON table_3.date = (
CASE
WHEN table_1.created_date > table_2.other_created_date THEN table_1.created_date
ELSE table_2.other_created_date
END
)
Basically, I'm interested in (Table_1 + Table_2) JOINed on Table_3, where if the first statement is true we join on Table_1's date, and if the second statement is true we join on Table_2's date
Is this possible or is there a better way to go at this?

Answer 1

SELECT * FROM table_1
JOIN table_2
ON table_1.id = table_2.table_1_fk
FULL OUTER JOIN table_3
ON table_3.date = GREATEST(table_1.created_date,table_2.other_created_date)

Answer 2

I like Bernd's answer. However, without knowing anything about the content of these tables, I think it's worth your while to evaluate the performance difference between doing what you are suggesting and simply having two separate outer joins. I know I've done creative things in the joins before, and the database will manage it, but HOW it manages it may not be what I had in mind at all, especially if dealing with tens of millions of records.
As an example, this is what the SQL might look like if you used two outer joins instead of trying to merge them into one. It will potentially be a lot more code, which is why you will need to benchmark it to see if it matters.
I know I used left joins here -- I'm always a little suspicious when I see a full outer, but that's not to say it wasn't exactly what you wanted. But this is for illustration purposes only:
SELECT
case
when table_1.created_date > table_2.other_created_date then
t3a.<field_1>
else
t3b.<field_1>
end
FROM
table_1
JOIN table_2
ON table_1.id = table_2.table_1_fk
left join table3 t3a on
table3.date = table_1.created_date
left join table3 t3b on
table3.date = table_2.other_created_date
-- EDIT --
Here's an example of where a compactly-coded join condition had horrible performance and a workaround that was way more code but worth it:
PostgreSQL Joining Between Two Values