Final answer:
To display data from two tables when one contains null values for a linking column, an Outer join should be used. It allows for the inclusion of all rows from one table and matches from another, plus rows with null values in the join column.
Step-by-step explanation:
The type of join you should use to display data from the EMPLOYEES table and the ORDERS table, especially when there are null values in the EMPLOYEE_ID column of the ORDERS table, is an Outer join. An Outer join will include all the records from the EMPLOYEES table and also those records from the ORDERS table where the EMPLOYEE_ID matches, as well as the rows where the EMPLOYEE_ID is null in the ORDERS table.
A Natural join would not be suitable because it only returns rows where there is a match in both tables, and would therefore exclude the ORDERS table rows with null EMPLOYEE_IDs. A Self-join is not applicable since it is used within a single table. An Equijoin, like a Natural join, only returns rows where there is a match on the specified columns in both tables.
The Outer join is further divided into three types: Left Outer join, Right Outer join, and Full Outer join. In this case, if you want to include all employees regardless of whether they have placed an order, you would use a Left Outer join. Conversely, if you want to include all orders, even those that do not have an associated employee, you would use a Right Outer join. If you want to include all employees and all orders, then you would use a Full Outer join.