How to: Create Outer Joins

By default, the Query and View Designer creates an inner join between tables. If you want to include data rows in the result set that do not have a match in the joined table, you can create an outer join.

When you create an outer join, the order in which tables appear in the SQL statement (as reflected in the SQL pane) is significant. The first table you add becomes the "left" table and the second table becomes the "right" table. (The actual order in which the tables appear in the Diagram pane is not significant.) When you specify a left or right outer join, you are referring to the order in which the tables were added to the query and to the order in which they appear in the SQL statement in the SQL pane.

NoteNote

Your computer might show different names or locations for some of the Visual Studio user interface elements in the following instructions. The Visual Studio edition that you have and the settings that you use determine these elements. For more information, see Customizing Development Settings in Visual Studio.

To create an outer join

  1. Create the join, either automatically or manually. For details, see How to: Join Tables Automatically or How to: Join Tables Manually.

  2. Select the join line in the Diagram pane, and then from the Query Designer menu, choose Select All Rows from <tablename>, selecting the command that includes the table whose extra rows you want to include.

    • Choose the first table to create a left outer join.

    • Choose the second table to create a right outer join.

    • Choose both tables to create a full outer join.

      Note Note

      Some databases, such as Oracle, do not support full outer joins.

When you specify an outer join, the Query and View Designer modifies the join line to indicate an outer join.

In addition, the Query and View Designer modifies the SQL statement in the SQL pane to reflect the change in join type, as shown in the following statement:

SELECT employee.job_id, employee.emp_id,
   employee.fname, employee.minit, jobs.job_desc
FROM employee LEFT OUTER JOIN jobs ON 
    employee.job_id = jobs.job_id

Because an outer join includes unmatched rows, you can use it to find rows that violate foreign key constraints. To do so, you create an outer join and then add a search condition to find rows in which the primary key column of the rightmost table is null. For example, the following outer join finds rows in the employee table that do not have corresponding rows in the jobs table:

SELECT employee.emp_id, employee.job_id
FROM employee LEFT OUTER JOIN jobs 
   ON employee.job_id = jobs.job_id
WHERE (jobs.job_id IS NULL)

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