Final answer:
The 'IN' operator in SQL is used to check for set membership, where the set consists of values resulting from a select clause. This operator is particularly useful in queries where a subquery generates the set of values to be checked for a specific value's presence.
Step-by-step explanation:
The connective that tests for set membership, where the set is a collection of values produced by a select clause, is the IN operator in SQL. When you have a set of values generated by a subquery, and you wish to check if a certain value belongs to this set, the IN operator allows you to determine if that value is a member of the set. For example, if we want to find all employees who are in a certain department, we could use the IN operator in a query like SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_name = 'Sales');.
In this SQL example, the subquery (SELECT department_id FROM departments WHERE department_name = 'Sales') produces a set of department IDs, and the main query checks which employees have a department_id that is within this set.