5.3. Outer Joins
Sometimes you want to join two tables and return all the rows in one table whether or not the second table contains a match on the join condition. This is known as performing an outer join between two tables. To illustrate why you would want to join two tables in this way, consider the EMPLOYEES and DEPARTMENTS tables for Scott's widget company. The EMPLOYEES table has a column called DEPARTMENT_ID, which can contain NULL values. If you were to join the two tables on the DEPARTMENT_ID column, the query would not return all employees. Conversely, if you had departments that did not have any employees, you would not see all of the departments represented in the query results either.
outer join
A join between two or more tables ...
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