SQL and EF Core Performance Interview Path

An interview-focused path for reasoning from query shape to indexes, execution plans, transactions, EF Core tracking, N+1 diagnosis, concurrency, and production measurement. SQL snippets are illustrative and database-specific behaviour is called out explicitly.

Follow the modules in order, complete each lesson, and use the linked practice tests to check your understanding.

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Module 1

2 lessons

1
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2
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Module 2

2 lessons

Module 3

2 lessons

Module 4

2 lessons

Choose a lesson

8 lessons across 4 modules

Lesson 1 of 8 | SQL Reasoning

Start with Cardinality and the Required Result Shape

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Outcomes

Reason about how many rows enter and leave each operator before proposing syntax or an index.

Core method

Write the required grain: one row per customer, order, day, or event. Identify filters, join multiplicity, grouping, ordering, and pagination. A one-to-many join can multiply rows before aggregation; DISTINCT may hide that bug while adding work. Push selective filters when semantics allow, project only needed columns, and preserve outer-join intent by placing predicates carefully.

Worked example

To return customers with order totals, aggregate orders by customer first or group after a correct join. If customers with zero orders must remain, use a left join and keep order filters in the join condition or a null-safe expression; a WHERE predicate on the right table can accidentally turn it into an inner join.

Practice and review checklist

State output grain.n- Estimate cardinality after each join.n- Check one-to-many multiplication.n- Distinguish filtering rows from filtering groups.n- Verify null and empty-set behaviour.

SQL and EF Core Performance Interview Path

Your progress

SQL Reasoning-

Indexes and Transactions-

Efficient EF Core-

Production Diagnosis-

Outcomes

Core method

Worked example

Practice and review checklist