What does # mean in sql?

In Oracle SQL, this is the deprecated outer join operator.

It is an outer join operator in sql server it would be *= and =*

In Oracle, the (+) specifies that the join is an outer join (instead of an inner join as this implicit join syntax usually implies).

Making it an outer join means that the row should be included in the results even if that particular item is null.

In old Oracle versions, this means an OUTER JOIN.