GROUP BY

GROUP BY groups rows that have the same value in one or more columns, so you can run aggregate functions per group.

Count customers per country

sql

### SELECT Country, COUNT(CustomerID)

### FROM Customers

### GROUP BY Country;

Result:

The rule

Every column in SELECT that is not an aggregate must appear in GROUP BY.

Group by multiple columns

sql

### SELECT Country, City, COUNT(CustomerID)

### FROM Customers

### GROUP BY Country, City;

Group with other aggregates

sql

### SELECT CategoryID, AVG(Price), MAX(Price)

### FROM Products

### GROUP BY CategoryID;

ORDER of execution

sql

### SELECT Country, COUNT(*)

### FROM Customers

### GROUP BY Country

### ORDER BY COUNT(*) DESC;

• SELECT ... FROM ...
• WHERE ... (filter rows)
• GROUP BY ... (group rows)
• ORDER BY ... (sort the groups)

TL;DR

• GROUP BY groups rows sharing the same value.
• Aggregates then run per group.
• Non-aggregate columns in SELECT must be in GROUP BY.
• It goes after WHERE, before ORDER BY.