SQL: Day-over-Day Change (LAG)

MediumDatabases

Description

> This problem is paper-only - see explanation for the canonical SQL solution. Given `DailyMetric(date, metric)` rows (one per date, sorted) for a single dashboard, return each row with `prev_metric` (the previous day's metric, or null for the first row) and `delta` (today minus yesterday, or null for the first). **For the auto-grader:** return `[{ date, metric, prev_metric, delta }]`.

Example 1

Input: rows = [{date:"2026-01-01",metric:100},{date:"2026-01-02",metric:120},{date:"2026-01-03",metric:90}]

Output: [{date:"2026-01-01",metric:100,prev_metric:null,delta:null},{date:"2026-01-02",metric:120,prev_metric:100,delta:20},{date:"2026-01-03",metric:90,prev_metric:120,delta:-30}]

Constraints

- 0 <= rows.length <= 10^4 - dates are unique and pre-sorted ascending.

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