Clarify the schema and what 'total cost' means (e.g., sum of product costs and transaction amounts). Then write a SQL query that combines data from both tables, likely using a JOIN or UNION ALL, and aggregate with SUM. Consider edge cases like missing matches or duplicate rows.
Pro tip: Always clarify ambiguous requirements before writing SQL—interviewers value candidates who ask about data relationships and definitions. Also, mention how you'd handle NULLs and duplicates to show production-level thinking.
Ask about the columns in each table, the relationship between them (e.g., product_id foreign key), and what 'total cost' encompasses (product cost, transaction amount, or both).
Decide whether to use INNER JOIN, LEFT JOIN, or UNION ALL based on whether you need to include products without transactions or transactions without products.
Construct the query: select the sum of the relevant cost columns, join the tables on the appropriate key, and use aggregation. If summing across both tables separately, use UNION ALL then SUM.
Consider NULLs, duplicate rows from joins, and whether to use COALESCE. Explain how you'd test the query with sample data.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Start by clearly defining the two tier models: fixed-tier (flat rate per tier based on total usage) and incremental-tier (graduated pricing where each tier's rate applies only to the units within that tier). Then walk through a concrete example with sample tiers and usage to compute total cost for each model, highlighting the key difference in calculation logic and the implications for billing systems.
Pro tip: Emphasize that incremental-tier pricing requires careful handling of tier boundaries and cumulative calculations, which can lead to off-by-one errors; mention that Stripe's own billing system likely uses a graduated approach, so understanding this distinction is crucial for building accurate and scalable pricing engines.
Clearly explain that fixed-tier pricing charges a single rate based on the total volume (e.g., first 100 units at $1, next 100 at $0.80, but all units charged at the rate of the tier the total falls into). Incremental-tier pricing charges different rates for different ranges of units (e.g., first 100 at $1, next 100 at $0.80, etc.).
Choose simple numbers: tiers like 0-100 units at $1/unit, 101-200 at $0.80/unit, 201+ at $0.50/unit. Assume usage of 150 units. This makes the difference obvious.
For fixed-tier, determine which tier the total usage falls into (150 units falls in the 101-200 tier), then multiply total usage by that tier's rate: 150 * $0.80 = $120. All units are charged at the same rate.
For incremental-tier, break usage into segments: first 100 units at $1 = $100, next 50 units at $0.80 = $40, total = $140. Each tier's rate applies only to units within that tier.
Explain that incremental-tier requires iterating through tiers and summing partial costs, which is more complex but fairer for customers. Fixed-tier is simpler but can cause sudden jumps in cost at tier boundaries. Mention edge cases like exactly at boundary, zero usage, and rounding.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.