Welcome to the second part of the SQL Practice Set in PostgreSQL. Now we will focus on aggregation and grouping. However, we'll start with a quick recap.

• SQL offers aggregate functions that can help with computing statistics: COUNT(), SUM(), MIN(), MAX(), AVG(), etc.
• In order to create more sophisticated statistics with aggregate functions, you have to use the GROUP BY clause. This groups together all rows that have the same values.
• You can use the HAVING clause, if you need to filter the result of an aggregate function.

Take a look at the example below. This query selects every client_id and the average value of all items a given client ordered. It takes into consideration only those clients who ordered at least twice.

SELECT 
  client_id,
  AVG(order_price)
FROM client_orders
GROUP BY client_id
HAVING count(client_id) >= 2

• The DISTINCT keyword eliminates duplicate results.
• In order to sort table rows, you need to use the ORDER BY clause. For example:

  SELECT 
    client_id,
    name
  FROM client_orders
  ORDER BY name DESC