Oracle "(+)" Operator

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I am checking some old SQL Statements for the purpose of documenting them and probably enhancing them.

The DBMS is Oracle

I did not understand a statement which read like this:

select ...
from a,b
where a.id=b.id(+)

I am confused about the (+) operator, and could not get it at any forums... (searching for + within quotes didn't work either).

Anyway, I used 'Explain Plan' of SQLDeveloper and I got an output saying that HASH JOIN, RIGHT OUTER, etc.

Would there be any difference if I remove the (+) operator at the end of the query? Does the database have to satisfy some condition (like having some indexes, etc.) before (+) can be used?? It would be greatly helpful if you can provide me a simple understanding, or some good links where I can read about this.

Thanks!

outer-join join oracle sql
4 Answers

In Oracle, (+) denotes the "optional" table in the JOIN. So in your query,

SELECT a.id, b.id, a.col_2, b.col_2, ...
FROM a,b
WHERE a.id=b.id(+)

it's a LEFT OUTER JOIN of table 'b' to table 'a'. It will return all data of table 'a' without losing its data when the other side (optional table 'b') has no data.

Diagram of Left Outer Join

The modern standard syntax for the same query would be

SELECT  a.id, b.id, a.col_2, b.col_2, ...
FROM a
LEFT JOIN b ON a.id=b.id

or with a shorthand for a.id=b.id (not supported by all databases):

SELECT  a.id, b.id, a.col_2, b.col_2, ...
FROM a
LEFT JOIN b USING(id)

If you remove (+) then it will be normal inner join query

Older syntax, in both Oracle and other databases:

SELECT  a.id, b.id, a.col_2, b.col_2, ...
FROM a,b
WHERE a.id=b.id

More modern syntax:

SELECT  a.id, b.id, a.col_2, b.col_2, ...
FROM a
INNER JOIN b ON a.id=b.id

Or simply:

SELECT  a.id, b.id, a.col_2, b.col_2, ...
FROM a
JOIN b ON a.id=b.id

Diagram of Inner Join

It will only return all data where both 'a' & 'b' tables 'id' value is same, means common part.

If you want to make your query a Right Join

This is just the same as a LEFT JOIN, but switches which table is optional.

Diagram of Right Outer Join

Old Oracle syntax:

SELECT  a.id, b.id, a.col_2, b.col_2, ...
FROM a,b
WHERE a.id(+)=b.id

Modern standard syntax:

SELECT  a.id, b.id, a.col_2, b.col_2, ...
FROM a
RIGHT JOIN b ON a.id=b.id

Ref & help:

https://asktom.oracle.com/pls/asktom/f?p=100:11:::::P11_QUESTION_ID:6585774577187

Left Outer Join using + sign in Oracle 11g

https://www.w3schools.com/sql/sql_join_left.asp

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