When you work with a lot of data, you often want to combine rows that have the same values in certain columns and calculate something for each group.
In SQL, this is done with the GROUP BY clause.

Imagine you have a sales table:

If you want total quantity sold for each category, you can group the rows by category.

SELECT column1, column2, ..., aggregate_function(column)
FROM table_name
GROUP BY column1, column2, ...;

• aggregate_function: Functions that calculate a value for a group, such as:

  • COUNT() → Counts rows
  • SUM() → Adds values
  • AVG() → Calculates average
  • MIN() → Finds smallest value
  • MAX() → Finds largest value

SELECT category, SUM(quantity) AS total_quantity
FROM sales
GROUP BY category;

Result:

How it works:

1. SQL looks at the category column.
2. Rows with the same category are grouped together.
3. The SUM(quantity) is calculated for each group.

You can group by more than one column.

SELECT category, product, SUM(quantity) AS total_quantity
FROM sales
GROUP BY category, product;

Now each unique (category, product) pair is its own group.

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

Example: Show only categories where total quantity > 10.

SELECT category, SUM(quantity) AS total_quantity
FROM sales
GROUP BY category
HAVING SUM(quantity) > 10;

Result: