A less commonly used variant of the inner join relational database operation. An inner join selects rows from two tables such that the value in one column of the first table also appears in a certain column of the second table. For an outer join, the result also includes all rows from the first operand ("left outer join"), or the second operand ("right outer join"), or both ("full outer join"). A field in a result row will be null if the corresponding input table did not contain a matching row.