Question 1

What is the difference between INNER JOIN and LEFT JOIN?

Accepted Answer

INNER JOIN returns only rows that match in both tables. LEFT JOIN returns every row from the left table and fills the right-side columns with NULL where there is no match. Reach for LEFT JOIN when 'customers with no orders' should still appear.

Question 2

How would you find the second-highest salary in a table?

Accepted Answer

Use a window function: SELECT DISTINCT salary FROM (SELECT salary, DENSE_RANK() OVER (ORDER BY salary DESC) rnk FROM employees) t WHERE rnk = 2. DENSE_RANK handles ties correctly; a bare LIMIT/OFFSET breaks when two people share the top salary.

Question 3

What does a p-value actually mean?

Accepted Answer

It is the probability of seeing a result at least this extreme if there were no real effect. A p of 0.03 does not mean a 97% chance the variant is better — it means a gap this big would occur 3% of the time by chance alone. It says nothing about effect size or business impact.

Question 4

Correlation vs causation — how do you explain the difference?

Accepted Answer

Correlation means two variables move together; causation means one drives the other. Ice-cream sales and drowning deaths correlate, but both are driven by hot weather. To claim causation you need a controlled experiment or a strong design that rules out the lurking variables.

Question 5

A key metric dropped 20% overnight. How do you investigate?

Accepted Answer

First rule out instrumentation — a broken tracking tag or a data pipeline failure explains most overnight cliffs. Then segment: by platform, region, new vs returning, and version. A drop concentrated in one segment points at a cause; a uniform drop points at tracking or an external event. State a hypothesis, then check it against the data.

Question 6

How do you explain a technical finding to a non-technical stakeholder?

Accepted Answer

Lead with the decision, not the method. Say what you found and what they should do, in one sentence, then offer the 'how' only if asked. Use their units (revenue, customers, days), not yours (p-values, joins). The best analysts are translators, and this question tests exactly that.

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