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Practice

GROUP BY

The GROUP BY clause lets you group rows that share the same values in specified columns. It's commonly used with aggregate functions like COUNT(), SUM(), and AVG() to summarize data by category.


Why Use GROUP BY?​

It allows you to answer questions like:

  • How many orders were placed in each region?
  • What's the total sales per client?
  • What's the average order value by region?

Basic Syntax​

The GROUP BY clause is used after the FROM clause in a SELECT statement.

GROUP BY Syntax
SELECT column_name, AGG_FUNCTION(column_name)
FROM table_name
GROUP BY column_name;

You group by a column, then apply an aggregate function to summarize each group.


좋습니다. 지금까지 만든 groupby_numeric.sqlite와 실습 SQL 코드를 바탕으로, 이어지는 교육 자료를 초보자도 따라 할 수 있도록 작성해 드리겠습니다. 아래 내용은 지금 작성하신 문서에 그대로 이어 붙이면 됩니다.

Step-by-Step Practice​

We will use the clients table, which contains:

client_idnameemailregionsalesorders_count
1Client 1client1@example.comTexas804119
2Client 2client2@example.comNew York86193
..................

Count Clients per Region​

By using GROUP BY with COUNT(*), we can count how many clients belong to each region.

-- Count how many clients there are in each region
SELECT region, COUNT(*) AS client_count
FROM clients
GROUP BY region;

Total Sales per Region​

By using GROUP BY with SUM(sales), we can calculate the total sales for each region.

-- Show total sales in each region
SELECT region, SUM(sales) AS total_sales
FROM clients
GROUP BY region;

Average Sales per Region​

By using GROUP BY with AVG(sales), we can calculate the average sales for each region.

-- Show average sales in each region
SELECT region, AVG(sales) AS avg_sales
FROM clients
GROUP BY region;

Key Takeaways​

  • GROUP BY groups rows with the same value in the specified column(s).
  • Aggregate functions like COUNT(), SUM(), and AVG() summarize each group.
  • The order of clauses matters:
    1. SELECT
    2. FROM
    3. GROUP BY
    4. ORDER BY (optional for sorting groups)

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