Learning PostgreSQL

Advanced SQL

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

• The DISTINCT ON clause, which helps finding the first records in groups
• The set returning functions, which are functions that return relations
• LATERAL joins, which allow subqueries to reference each other
• 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 will require a subquery to implement the logic:

SELECT advertisement_id, advertisement_date, adv.car_id,
    seller_account_id
 FROM car_portal_app.advertisement ...