SELECT col1_name, SUM(col2_name)
FROM table_name
GROUP BY col1_name;
-- `GROUP BY` is used with aggregate functions to perform calculations on the grouped data
-- every column in the SELECT clause must either be listed in the GROUP BY clause or be used within an aggregate function

SELECT column1, column2, AVG(column3)
FROM table_name
GROUP BY column1, column2;
-- can `GROUP BY` multiple cols

/* Write your T-SQL query statement below */

SELECT player_id, MIN(event_date) AS first_login
FROM Activity
GROUP BY player_id;

/* Write your T-SQL query statement below */

SELECT TOP 1 customer_number
FROM Orders
GROUP BY customer_number
ORDER BY COUNT(*) DESC;

/* Write your T-SQL query statement below */

SELECT [user_id], COUNT(follower_id) AS followers_count
FROM Followers
GROUP BY [user_id];

/* Write your T-SQL query statement below */

SELECT event_day AS [day], emp_id, SUM(out_time - in_time) AS total_time
FROM Employees
GROUP BY emp_id, event_day;