SQL

Most real-world data starts in a database. Being able to pull exactly what you need with SQL is often the first step before any ML work even begins.

Interview mindset:

For every query: What table(s) do I need? What am I filtering, grouping, or joining? What would break if there are duplicates or NULLs?

SELECT & WHERE

The basic building blocks — choosing columns and filtering rows.

Interview check:Know the difference between filtering with WHERE(before grouping) and HAVING (after grouping).

SELECT name, age
  FROM users
  WHERE age > 25;

JOINs

Combine rows from two or more tables based on a related column.

Interview check:Know the difference between INNER, LEFT, and RIGHT joins — and what happens to unmatched rows in each.

SELECT orders.id, users.name
  FROM orders
  LEFT JOIN users
    ON orders.user_id = users.id;

GROUP BY & Aggregations

Groups rows sharing a value and computes something across each group — counts, sums, averages.

Interview check:Know why every non-aggregated column in SELECT must appear in GROUP BY.

SELECT department, AVG(salary)
  FROM employees
  GROUP BY department;

Subqueries

A query nested inside another — useful for filtering based on a computed value.

Interview check:Know when a subquery could be rewritten as a JOIN, and why that might be more efficient.

SELECT name FROM users
  WHERE id IN (
    SELECT user_id FROM orders
    WHERE total > 100
  );

Indexes

A data structure that speeds up lookups on a column, at the cost of extra storage and slower writes.

Interview check:Explain why indexes speed up reads but can slow down inserts/updates.

CREATE INDEX idx_user_id
  ON orders (user_id);

SQL Interview Questions You Should Be Able to Answer

What is the difference between WHERE and HAVING?
What is the difference between INNER JOIN and LEFT JOIN?
What does GROUP BY actually do?
When would you use a subquery instead of a JOIN?
What is an index, and what's the tradeoff of using one?
What is the difference between COUNT(*) and COUNT(column)?

The Standard You Want

You should be able to write a query from a plain-English question without hesitating, and explain what happens to your result if the data has duplicates or missing values.

A working query isn't enough — know why it returns exactly what it does.