Add a column to the Employees table that shows the name of the employee with the next higher employee_id.

To add a column showing the name of the employee with the next higher employee_id, utilize the LEAD window function. This powerful function allows you to access data from a subsequent row within the same result set, based on a specified order.

The LEAD function's syntax is LEAD(column_name, offset, default_value) OVER (ORDER BY column_to_order_by). For this problem, LEAD(name) implicitly uses an offset of 1 (looking one row ahead) and a default NULL for the last row (when no subsequent row exists). The OVER (ORDER BY employee_id) clause is critical: it defines the window of rows and specifies the order in which LEAD should look for the "next" row. Ordering by employee_id ensures we correctly identify the employee with the next higher ID. This is a common pattern, with LAG serving the inverse purpose for previous rows.

SELECT
    employee_id,
    name,
    LEAD(name) OVER (ORDER BY employee_id) AS next_employee_name
FROM
    Employees;

This window function approach is generally the most performant and idiomatic for "next/previous row" problems. It typically involves a single pass over the data, requiring a sort operation. For very large tables, this sort can be resource-intensive, potentially leading to disk spills if the data exceeds memory capacity, similar to data shuffling in distributed systems like Spark. Modern database optimizers, such as those in Snowflake or PostgreSQL, are highly optimized to handle window functions, sometimes leveraging existing indexes on the ORDER BY column to reduce sorting costs, though a full sort is often unavoidable.

In the interview, also mention:

Explain that LEAD often includes a PARTITION BY clause within the OVER statement (e.g., OVER (PARTITION BY department_id ORDER BY employee_id)) to find the next employee within each department. Briefly discuss less efficient alternatives like a self-join (e.g., LEFT JOIN Employees e2 ON e1.employee_id = e2.employee_id - 1), highlighting why LEAD is preferred for its clarity and performance.