SQL JOINs & Aggregation Explained (INNER, LEFT, GROUP BY)

JOINs & Aggregation

1 min readPublished 22 Jul 2026

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Real questions span multiple tables. JOINs combine them; aggregation summarises the results.

Interactive explainer

SQL JOINs & Aggregation

Combining tables and summarising data.

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INNER JOIN

Only matches.

Returns only rows that have a match in both tables. Use it when you want records that exist on both sides — e.g. customers who have placed orders.

Check your understanding

1. Which JOIN returns all left-table rows plus matches?

2. What filters groups after aggregation?

Keep learning

JOINs & Aggregation Keys & Relationships Microsoft DP-900

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JOIN types

INNER JOIN — only rows with a match in both tables.
LEFT JOIN — all rows from the left table, plus matches (NULLs where none) — great for "customers with no orders".
RIGHT / FULL JOIN — the mirror and the union of both.

Aggregation

SELECT customer_id, COUNT(*), SUM(total) FROM orders GROUP BY customer_id; — GROUP BY buckets rows and functions (COUNT, SUM, AVG, MIN, MAX) summarise each bucket; HAVING filters the groups. JOINs rely on keys.

Interview Intelligence

How this topic actually shows up in interviews — and how to demonstrate you understand it.

Why employers ask about this

JOINs and aggregation are the heart of analytics; nearly every SQL interview tests them.

Technical questions

How would you find customers with no orders?+

LEFT JOIN customers to orders and filter WHERE orders.id IS NULL.

Behavioural questions

Tell me about turning data into a useful answer.+

Show framing the question, joining/aggregating and communicating the result.

Real-world scenarios

“You need total revenue per customer.”+

Expected answer: JOIN customers and orders, GROUP BY customer, SUM the totals, and HAVING to filter high-value ones.

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Employability Intelligence

Where this knowledge takes you — the jobs, skills and certifications it feeds into.

Relevant roles

Data AnalystBI DeveloperBackend Developer

Skills you're proving

JOINsGROUP BYAggregation

Recommended certifications

Microsoft DP-900 (Data Fundamentals)Oracle / SQL certifications

Career progression

Databases & SQL → analyst/BI roles.

What employers expect

That you can design a sensible schema and write correct, efficient SQL.

Frequently asked questions

What is the difference between INNER and LEFT JOIN?

INNER returns only matching rows in both tables; LEFT returns all left-table rows plus matches (NULLs where none).

What does GROUP BY do?

It groups rows sharing a value so aggregate functions (COUNT, SUM, AVG) can summarise each group.

What is the difference between WHERE and HAVING?

WHERE filters rows before grouping; HAVING filters groups after aggregation.

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