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:
- 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.
GROUP BY client_id
HAVING count(client_id) >= 2