SQL GROUP BY Explained: Common Questions and Clear Answers

[Rajeev Bagra]

Context

Many learners get confused about how GROUP BY behaves in SQL—especially when aggregate functions like MIN() or COUNT() are involved. A frequent misunderstanding is whether GROUP BY can return multiple rows per group when aggregates are not used.

This Q&A clears that confusion once and for all.

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Q1. What is the purpose of GROUP BY in SQL?

Answer:
GROUP BY is used to combine multiple rows into a single group based on one or more columns, so that you can compute one result per group.

In simple terms:

“Group rows that share the same value, then summarize them.”

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Q2. Does GROUP BY always return one row per group?

Answer:
Yes. Always.

This is the most important rule to remember:

👉 GROUP BY always produces exactly one row per group.

This rule does not change based on whether you use aggregate functions or not.

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Q3. What happens when GROUP BY is used with aggregate functions?

Answer:
Aggregate functions (such as MIN, MAX, COUNT, SUM, AVG) are applied once per group, producing one summary value per group.

Example:

SELECT year, MIN(air_date)
FROM episodes
GROUP BY year;

Result:

• One row per year
• Each row summarizes that year’s data

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Q4. What happens if I use GROUP BY without any aggregate function?

Answer:
You still get one row per group, but:

• In standard SQL, SQLite, PostgreSQL → ❌ Query fails with an error
• In MySQL (non-strict mode) → ⚠️ One arbitrary row is chosen per group

You do not get multiple rows per group.

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Q5. Why does SQL throw an error without aggregates?

Answer:
Because SQL cannot decide which row’s value to show for columns that are neither:

• Part of the GROUP BY clause, nor
• Wrapped in an aggregate function

Example (invalid):

SELECT year, title
FROM episodes
GROUP BY year;

SQL asks:

“Which title should I display for this year?”

Since there may be many, the query is rejected.

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Q6. Can one group ever contain multiple rows in the result?

Answer:
No. Never.

Once GROUP BY is applied, the result set contains:

• One row per group
• No exceptions

If you want multiple rows per group, you should not use GROUP BY.

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Q7. Then how do I keep multiple rows while still grouping by a column?

Answer:
Use alternatives instead of GROUP BY, such as:

1. ORDER BY (keeps all rows)

SELECT year, title
FROM episodes
ORDER BY year;

2. Window functions (advanced)

SELECT
  title,
  year,
  MIN(air_date) OVER (PARTITION BY year)
FROM episodes;

This keeps all rows but still computes group-level values.

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Q8. What is a good mental model for understanding GROUP BY?

Answer:
Think of GROUP BY as:

“Collapse many rows into one summary row per group.”

If SQL allowed multiple rows per group, grouping would lose its meaning.

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Q9. What is the golden rule to remember about GROUP BY?

Answer:

👉 Every column in SELECT must be either:

1. Listed in GROUP BY, or
2. Wrapped in an aggregate function

Otherwise, the query is invalid (or unreliable in some databases).

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Final Takeaway

Situation	Result
GROUP BY used	One row per group
Aggregate used	One row per group
No aggregate	Error or arbitrary row
Want many rows	Do not use GROUP BY

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