Final answer:
The assertion that correlated subqueries perform tasks that cannot be replicated using joins is true. Correlated subqueries are executed for each row of the outer query and are particularly useful for row-specific operations that cannot be achieved with standard join operations.
Step-by-step explanation:
The statement that correlated subqueries do work that cannot be duplicated as a join is true. A correlated subquery is a type of SQL query where the subquery depends on the outer query for its values. This means that the subquery uses values from the outer query as part of its condition, and it is executed repeatedly, once for each row that is being evaluated in the outer query.
While it's true that many subqueries can be rewritten as joins to optimize performance, there are certain conditions, particularly when evaluating individual row-level operations or aggregations that are dependent on each specific row of the main query, where correlated subqueries are the more appropriate or the only choice. Joins are not always an alternative in these cases because they do not offer the same level of row-by-row processing capabilities that correlated subqueries provide. An example of a situation where a correlated subquery is necessary is when we want to select rows from a table where a column value is equal to the maximum value for that column within a group defined by another column.