Explain the difference between an inner join and an outer join in SQL.

An inner join returns only the records that have matching values in both tables, while an outer join returns all the records from one table and the matched records from the second table.

In SQL, a join operation combines rows from two or more tables based on a related column between them. The two most common types of joins are inner join and outer join. An inner join is a type of join that returns only the rows where there is a match in both tables. If there is a row in the first table that matches a row in the second table, the inner join will include that row in the result set. If there is no match, the row is not included in the result set. This means that an inner join only returns the intersection of the two tables.

On the other hand, an outer join is a type of join that returns all the rows from one table and the matched rows from the second table. If there is no match, the result is NULL on the side of the table that doesn't have a match. There are three types of outer joins: left outer join (returns all the rows from the left table and the matched rows from the right table), right outer join (returns all the rows from the right table and the matched rows from the left table), and full outer join (returns all rows for which there is a match in either of the tables).

In essence, the key difference between an inner join and an outer join lies in the result set they return. An inner join will only return rows where there is a match in both tables, whereas an outer join will return all rows from one table and any matching rows from another table. This means that an outer join can return more rows than an inner join. The choice between using an inner join or an outer join in SQL depends on the specific requirements of your query.