An outer join defines a relationship between two tables where all records from one or both tables are returned regardless of the existence of a matching key-field in the other table. A full outer join combines the results of both tables. A left or right join returns all the records from the first or second specified table, respectively. NULLS are filled in for matches on either side. A self-join compares a table to a copy of itself.

- Wiki
1 articles, 0 books.

An outer join is used to match rows from two tables. Even if there is no match rows are included. Rows from one of the tables are always included, for the other, when there are no matches, NULL values are included.