Final answer:
The outer join operator is placed on the side of the joining condition with non-matching rows. It includes left, right, and full outer joins to combine rows from two or more tables even if there's no matching row on one side of the join.
Step-by-step explanation:
The outer join operator is used in SQL to combine rows from two or more tables. This operator is placed on the side of the joining condition that has non-matching rows. When using an outer join, you can specify a left outer join, right outer join, or full outer join, depending on which table's non-matching rows you want to include in the result set:
In a left outer join, all rows from the left table are included, along with the matched rows from the right table. The outer join operator is placed on the right table side that may have non-matching rows.
In a right outer join, it's the opposite: all rows from the right table are included regardless of whether there are matching rows in the left table. Here the outer join operator is placed on the left side.
For a full outer join, all rows from both tables are included, whether they have matching rows in the opposing table or not.
Using the outer join ensures that even if there's no matching row on one side of the join, the query will still return the row from the other table with NULL values for the columns from the table without a match.