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Multi-column grouping

Group by multiple columns.

Multi-column grouping

Grouping by multiple columns creates subgroups. Column order matters.

SELECT country, city, COUNT(*)
FROM customers
GROUP BY country, city;

This query counts by country and city.

country, city, COUNT(*) is selected from customers. Rows are grouped by country, city; aggregate functions are computed per group.

Multi-column grouping — details

The first column is the parent group, the second is the subgroup. Each distinct pair is a row.

For example, GROUP BY country, city gives a separate row for each city in each country: Türkiye-İzmir and Türkiye-Bursa are separate groups.

Practice Multi-column grouping

Group customers first by country then by city.

Starter code

query.sql
-- country city
SELECT country, city, COUNT(*)
FROM customers
GROUP BY country

Sample solution

query.sql
SELECT country, city, COUNT(*)
FROM customers
GROUP BY country, city;

Expected output

country  city       count
Türkiye  İstanbul   10
Türkiye  Ankara     6

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Multi-column grouping: match the terms

Term Meaning
country country
city city
subgroup child
GROUP BY group

2 Add the second column

Question 
SELECT country, city, COUNT(*)
FROM customers
GROUP BY country, ____;
Answer 
SELECT country, city, COUNT(*)
FROM customers
GROUP BY country, city;

Explanation: GROUP BY country, city creates a separate group for each country and city combination.

3 Does order matter in multi-column GROUP BY?

  • Yes Correct answer
  • No
  • In some cases
  • Sadece COUNT'ta

Explanation: Column order sets the group hierarchy.

All lessons in Intermediate

1 INNER JOIN in depth

  1. INNER JOIN logic
  2. Choosing the join column
  3. Three-table INNER JOIN
  4. Self JOIN
  5. Filtering joins
  6. Summary - INNER JOIN

2 LEFT and RIGHT JOIN

  1. LEFT JOIN
  2. RIGHT JOIN
  3. Unmatched rows
  4. JOINs with NULL
  5. LEFT vs RIGHT
  6. Summary - LEFT and RIGHT JOIN

3 FULL and CROSS JOIN

  1. FULL OUTER JOIN
  2. CROSS JOIN
  3. When to CROSS JOIN
  4. Comma JOIN
  5. Choosing a JOIN type
  6. Summary - FULL and CROSS JOIN

4 Project: Library database

  1. Library design
  2. Books and authors
  3. List loans with LEFT JOIN
  4. Find overdue loans
  5. Member summary

5 GROUP BY

  1. What is GROUP BY?
  2. Single-column grouping
  3. Multi-column grouping
  4. GROUP BY with aggregates
  5. Ordering grouped results
  6. Summary - GROUP BY

6 HAVING

  1. What is HAVING?
  2. HAVING vs WHERE
  3. HAVING with aggregates
  4. Multiple conditions
  5. HAVING with aliases
  6. Summary - HAVING

7 Aggregate functions

  1. COUNT
  2. SUM
  3. AVG
  4. MIN and MAX
  5. NULL and aggregates
  6. Summary - Aggregates

8 Project: Sales report

  1. Sales design
  2. Daily revenue
  3. Best sellers
  4. Customer segments
  5. Low performers

9 Subquery

  1. What is a subquery?
  2. Scalar subquery
  3. Subquery with IN
  4. ANY and ALL
  5. Correlated subquery
  6. Summary - Subquery

10 EXISTS

  1. What is EXISTS?
  2. NOT EXISTS
  3. EXISTS vs IN
  4. EXISTS performance
  5. Multiple EXISTS
  6. Summary - EXISTS

11 UNION and INTERSECT

  1. UNION
  2. UNION ALL
  3. INTERSECT
  4. EXCEPT
  5. Set rules
  6. Summary - UNION and INTERSECT

12 CASE

  1. What is CASE?
  2. Searched CASE
  3. Simple CASE
  4. The ELSE branch
  5. CASE with aggregates
  6. Summary - CASE

13 VIEW

  1. What is a VIEW?
  2. CREATE VIEW
  3. Querying a VIEW
  4. DROP VIEW
  5. Updatable VIEWs
  6. Summary - VIEW

14 INDEX basics

  1. What is an INDEX?
  2. CREATE INDEX
  3. Composite INDEX
  4. DROP INDEX
  5. When to index
  6. Summary - INDEX

15 Project: Blog database

  1. Blog design
  2. Post list
  3. Tag search
  4. Author stats
  5. Search index

16 Normalization

  1. What is normalization?
  2. First normal form
  3. Second normal form
  4. Third normal form
  5. BCNF
  6. Summary - Normalization

17 Data integrity

  1. What is integrity?
  2. Entity integrity
  3. Referential integrity
  4. Domain integrity
  5. Integrity with triggers
  6. Summary - Data integrity

18 Transaction basics

  1. What is a transaction?
  2. BEGIN and COMMIT
  3. ROLLBACK
  4. SAVEPOINT
  5. ACID
  6. Summary - Transaction

19 ALTER and DROP

  1. ALTER TABLE
  2. Adding a column
  3. Dropping a column
  4. Changing a column type
  5. DROP TABLE
  6. Summary - ALTER and DROP

20 Intermediate final

  1. Multi-JOIN report
  2. Aggregate analysis
  3. Subquery challenge
  4. Transaction scenario