Month-Over-Month Revenue Change

Problem

Schema: Revenue(month VARCHAR, total DECIMAL). One row per YYYY-MM. Return month, total, prev_total, and pct_change = (total - prev) / prev * 100 rounded to 2 decimals. First row has NULL for prev and change.

Constraints

• Months are sortable as text (YYYY-MM format).

Approach — Core Patterns

This is a Core Patterns problem. The idea: pick the data structure best matched to the constraints and solve it in one clean pass. Work through the reference code below line by line, then re-derive it yourself in the editor — that's how the pattern sticks.

Complexity: see the walkthrough below.

Solution code

SQL

SELECT month, total,
       LAG(total) OVER (ORDER BY month) AS prev_total,
       ROUND(
         (total - LAG(total) OVER (ORDER BY month)) * 100.0
         / NULLIF(LAG(total) OVER (ORDER BY month), 0),
         2
       ) AS pct_change
FROM Revenue
ORDER BY month;

Practice it

Reading a solution isn't the same as being able to write it under pressure. Open this problem in the in-browser editor, hide the solution, and solve it from scratch — your code runs against real test cases instantly.