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#0330
Previous Order Amount
MediumSQL24 min20 XP
Problem
Return user_id, order_date, amount, and previous_amount for each order.
Why This Matters
SQL interviews usually test whether you can turn a business question into the right rows, grouping level, join type, and edge-case behavior.
Schema
orders(order_id INTEGER, user_id INTEGER, order_date TEXT, amount INTEGER)
Examples
Example 1
Inputuser 1 has two orders
Outputsecond order previous_amount is 50
LAG reads the prior row inside the same user partition.
Constraints
- Return only the requested columns.
- Use deterministic ORDER BY when multiple rows are returned.
- Assume SQLite-compatible SQL unless the prompt states otherwise.
CodeSQL
Visible browser tests run here when available.
Testcases1 visible / 3 hidden categories
Core SQL case
Inputuser 1 has two orders
Expected[[1,"2026-01-01",50,null],[1,"2026-01-03",80,50],[2,"2026-01-02",40,null]]
LAG reads the prior row inside the same user partition.
Hidden Test Categories
Empty or missing matchesDuplicate rows or tiesNULL values where the pattern commonly breaks