What does the symbol (+) do in Oracle SQL? [duplicate]

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What does (+) do in Oracle SQL?

Oracle (Old?) Joins - A tool/script for conversion?

I'm new to SQL, I'm currently taking up SQL fundamentals so please bear with me.. I'm just wondering what the (+) symbol signifies in SQL. please see below:

CREATE VIEW title_avail AS
SELECT t.title, c.copy_id, c.status开发者_开发百科, r.exp_ret_date
FROM title t, title_copy c, rental r
WHERE t.title_id = c.title_id
AND c.copy_id = r.copy_id(+)
AND c.title_id = r.title_id(+);