Medium · Database

Latest Order Per Customer

Given Orders(id, customerId, placedAt), return CustomerId, OrderId, and PlacedAt for each customer’s latest order. Dates are ISO YYYY-MM-DD strings; break same-day ties by larger order ID. Row order does not matter.

Examples

Example 1

{
  "orders": [
    {"id": 10, "customerId": 1, "placedAt": "2026-01-01"},
    {"id": 11, "customerId": 1, "placedAt": "2026-01-03"},
    {"id": 12, "customerId": 1, "placedAt": "2026-01-03"},
    {"id": 20, "customerId": 2, "placedAt": "2026-01-02"}
  ]
}

Output: [{"CustomerId":1,"OrderId":12,"PlacedAt":"2026-01-03"},{"CustomerId":2,"OrderId":20,"PlacedAt":"2026-01-02"}]

Example 2

{
  "orders": [
    {"id": 99, "customerId": 7, "placedAt": "2025-12-31"},
    {"id": 3, "customerId": 7, "placedAt": "2026-01-01"}
  ]
}

Output: [{"CustomerId":7,"OrderId":3,"PlacedAt":"2026-01-01"}]

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