outerjoin

An outer join is a type of join operation in relational databases that combines rows from two tables based on a related column between them. Unlike an inner join, which only returns rows where there is a match in both tables, an outer join includes all rows from one or both tables, even if there is no match in the other table.

There are three main types of outer joins: left outer join, right outer join, and full outer join. A left outer join returns all rows from the "left" table and the matched rows from the "right" table. If there is no match in the right table for a row in the left table, the columns from the right table will contain null values. A right outer join works in the opposite way, returning all rows from the "right" table and matched rows from the "left" table, with nulls for unmatched rows from the left. A full outer join returns all rows from both tables. If a row from one table has no match in the other, the columns from the unmatched table will contain null values.

Outer joins are useful for scenarios where you need to retrieve all data from one table and supplement it with related data from another table, or when you want to identify records that do not have corresponding entries in another table. They are commonly used in SQL and other database query languages.