Database SQL

Avoid double-counting payments and refunds

Pre-aggregates two one-to-many relationships before joining them to prevent fanout from corrupting financial totals.

Build daily order KPIs with conditional aggregation

Builds several daily business metrics in one grouped query while keeping each metric definition explicit.

Calculate a running account balance

Uses an ordered window aggregate to calculate a deterministic running balance per account.

Calculate seven-day rolling revenue

Combines daily aggregation, a calendar table, and a window frame to calculate true seven-calendar-day revenue.

Choose isolation for a concurrent reservation

Chooses constraints, conditional writes, optimistic conflicts, or stronger isolation to protect a limited-resource reservation without unnecessary blocking.

Compare covering indexes and wide indexes

Designs a narrow covering index from predicates and ordering, then weighs eliminated lookups against storage, write overhead, cache efficiency, overlap, and maintenance cost.

SQL Practical Query Problems

Hands-on SQL rounds built around real product data and production constraints.

A practical SQL interview track covering joins, anti-joins, many-to-many filters, interval overlap, aggregation, fanout-safe totals, window functions, stable pagination, hierarchy queries, SARGable dates, and concurrent updates.

Database SQL

SQL querying, data modeling, performance, and consistency.

A practical SQL interview track covering query writing, relational modeling, execution plans, indexes, transactions, locking, migrations, and production database behavior.