Types of Joins

When you join tables, the type of join that you create determines the rows that appear in the result set. You can create the following types of joins:

• Inner join   A join that displays only the rows that have a match in both joined tables. (This is the default type of join in the Query Designer.) For example, you can join the titles and publishers tables to create a result set that shows the publisher name for each title. In an inner join, titles for which you do not have publisher information are not included in the result set, nor are publishers with no titles.

  Note   Columns containing NULL do not match any values when you are creating an inner join and are therefore excluded from the result set. Null values do not match other null values.

• Outer join   A join that includes rows even if they do not have related rows in the joined table. You can create three variations of an outer join to specify the unmatched rows to be included:
  • Left outer join   All rows from the first-named table (the "left" table, which appears leftmost in the JOIN clause) are included. Unmatched rows in the right table do not appear. For example, the following SQL statement illustrates a left outer join between the titles and publishers tables to include all titles, even those you don’t have publisher information for:

    SELECT titles.title_id, titles.title,
       publishers.pub_name
    FROM titles LEFT OUTER JOIN publishers ON 
       titles.pub_id = publishers.pub_id
    

  • Right outer join   All rows in the second-named table (the "right" table, which appears rightmost in the JOIN clause) are included. Unmatched rows in the left table are not included. For example, a right outer join between the titles and publishers tables will include all publishers, even those who have no titles in the titles table.
    
    
  • Full outer join   All rows in all joined tables are included, whether they are matched or not. For example, a full outer join between titles and publishers shows all titles and all publishers, even those that have no match in the other table.

    Note   Some databases, such as Oracle, do not support full outer joins. For details, see Query Designer Considerations for Oracle Databases.

You can also create a self-join, which compares rows within the same table. For example, you can use a self-join to find all publishers that are in a specific city and have the same postal code. For details, see Creating Self-Joins.