Final answer:
An outer join operator is used within a single SELECT statement to combine rows from multiple tables, with LEFT JOIN and RIGHT JOIN being types of outer joins. these join types include all rows from one table and matched rows from the other filling in with NULLs for unmatched rows.
Step-by-step explanation:
The outer join operator is not used to combine the results of multiple SELECT statements as indicated in the question; instead, it is used within a single SELECT statement to combine rows from two or more tables based on a related column between them. the correct answer to the question 'The outer join operator is used to combine the results of multiple SELECT statements' is none of the options provided (a) INNER JOIN, (b) LEFT JOIN, (c) RIGHT JOIN, (d) CROSS JOIN. However, amongst the given options, LEFT JOIN and RIGHT JOIN are types of outer joins.An INNER JOIN only combines rows from different tables where the join condition is met.
A CROSS JOIN, on the other hand, produces a Cartesian product of the two tables, combining every row of the first table with every row of the second table.The outer joins, LEFT JOIN and RIGHT JOIN, include all rows from one table and matched rows from the other table. If there are no matches, NULL values are used to fill in the columns from the table that does not have a matching row.