Advanced SQL techniques

In the following section, some other advanced SQL techniques will be introduced:

• The DISTINCT ON clause, which helps in finding the first records in groups
• Selecting sample data from a big table
• The set returning functions, which are functions that return relations
• LATERAL joins, which allow subqueries to reference each other
• Advanced grouping techniques that can be used in generating reports
• Some special aggregating functions

Selecting the first records

Quite often, it is necessary to find the first records based on some criteria. For example, let's take the car_portal database; suppose it is required to find the first advertisement for each car_id in the advertisement table.

Grouping can help in this case. It requires a subquery to implement the logic:

SELECT advertisement_id, advertisement_date, adv.car_id, seller_account_id
 FROM car_portal_app.advertisement adv
 INNER JOIN
 (SELECT car_id, min(advertisement_date) min_date FROM car_portal_app.advertisement GROUP BY car_id...