Creating Self-Joins Manually

Creating Self-Joins Manually

You can join a table to itself even if the table does not have a reflexive relationship in the database.  For example, you can use a self-join to find pairs of authors living in the same city.

As with any join, a self-join requires at least two tables. The difference is that, instead of adding a second table to the query, you add a second instance of the same table. That way, you can compare a column in the first instance of the table to the same column in the second instance, which allows you to compare the values in a column to each other. The Query Designer assigns an alias to the second instance of the table.

For example, if you are creating a self-join to find all pairs of authors within Berkeley, you compare the city column in the first instance of the table against the city column in the second instance. The resulting query might look like the following:

   SELECT 
         authors.au_fname, 
         authors.au_lname, 
         authors1.au_fname AS Expr2, 
         authors1.au_lname AS Expr3
      FROM 
         authors 
            INNER JOIN
            authors authors1 
               ON authors.city 
                = authors1.city
      WHERE
         authors.city = 'Berkeley'

Creating a self-join often requires multiple join conditions. To understand why, consider the result of the preceding query:

   Cheryl Carson       Cheryl Carson
   Abraham Bennet      Abraham Bennet
   Cheryl Carson       Abraham Bennet
   Abraham Bennet      Cheryl Carson

The first row is useless; it indicates that Cheryl Carson lives in the same city as Cheryl Carson.  The second row is equally useless.  To eliminate this useless data, you add another condition retaining only those result rows in which the two author names describe different authors.  The resulting query might look like this:

   SELECT 
         authors.au_fname, 
         authors.au_lname, 
         authors1.au_fname AS Expr2, 
         authors1.au_lname AS Expr3
      FROM 
         authors 
            INNER JOIN
            authors authors1 
               ON authors.city 
                = authors1.city
               AND authors.au_id
                <> authors1.au_id
      WHERE
         authors.city = 'Berkeley'

The result set is improved:

   Cheryl Carson       Abraham Bennet
   Abraham Bennet      Cheryl Carson

But the two result rows are redundant.  The first says Carson lives in the same city as Bennet, and the second says the Bennet lives in the same city as Carson.  To eliminate this redundancy, you can alter the second join condition from "not equals" to "less than".  The resulting query might look like this:

   SELECT 
         authors.au_fname, 
         authors.au_lname, 
         authors1.au_fname AS Expr2, 
         authors1.au_lname AS Expr3
      FROM 
         authors 
            INNER JOIN
            authors authors1 
               ON authors.city 
                = authors1.city
               AND authors.au_id
                < authors1.au_id
      WHERE
         authors.city = 'Berkeley'

And the result set looks like this:

   Cheryl Carson       Abraham Bennet

To create a self-join manually

1. Add to the Diagram pane the table or table-structured object you want to work with.
   
   
2. Add the same table again, so that the Diagram pane shows the same table or table-structured object twice within the Diagram pane..

   The Query Designer assigns an alias to the second instance by adding a sequential number to the table name. In addition, the Query Designer creates a join line between the two occurrences of the table or table-structured object within the Diagram pane.

3. Right-click the join line, choose Properties from the shortcut menu, and then change the comparison operator between the primary keys as required. For example, you might change the operator to less than (<).
   
   
4. Create the additional join condition (for example, authors.zip = authors1.zip) by dragging the name of the primary join column in the first occurrence of the table or table-structured object and dropping it on the corresponding column in the second occurrence.
   
   
5. Specify other options for the query such as output columns, search conditions, and sort order.

See Also

Querying Using Multiple Tables