outer join

Can anyone explain to me in a few sentences where exactly to place the (+) of an outer join in Oracle?

Take the following situation:

Ther is a maintable MAIN, containing an id main_id and foreign keys to secundairy tables, e.g. MAIN.sub_id points to SUBTABLE which contains the field SUBTABLE.text.
If I want to print all the records of MAIN with their corresponding texts, I could write:

SELECT m.main.id, s.text
FROM MAIN m, SUBTABLE s
WHERE m.sub_id = s.sub_id

But this will only show those records where m.sub_id is not NULL. If I want to show all records, should I write WHERE m.sub_id (+) = s.sub_id
or
WHERE m.sub_id = s.sub_id (+)?

I think the latter. But why exactly?
Would it be a useable rule-of-thumb to state that in cases like this, the (+) must be placed next to the Primary key?

Thanks for any help,
Michiel

-- 
Michiel Perdeck


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