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outer join

outer join in Technology

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.
For example, if we want to list all employees and their employee number, but not all employees have a number, then we could say (in SQL-92 syntax, as used by Microsoft SQL Server):
SELECT, empnum.number FROM employee LEFT JOIN empnum ON =
or, in Sybase syntax:
SELECT, empnum.number FROM employee, empnum WHERE *=
The "*" on the left means "left outer join". "*=*" would be a full outer join.
In Oracle syntax:
SELECT, empnum.number FROM employee, empnum WHERE = (+)
Note that the "(+)" on the right means "left outer join".
These all mean that all rows from the left-hand "employee" table will appear in the result, even if there is no match for their ID in the empnum table. Where there is no equal to a given, a result row is output anyway but with all result columns from the empnum table null (empnum.number in this case).

The Free On-line Dictionary of Computing, © Denis Howe 2010
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