In Business Central, a Query Object is a way to extract data from one or more tables. This is done in a very performance-friendly way. The output is a flat data struture.

A Query can be used in two different ways:

1. As an API
2. As an on-screen Report(*)

You can join multiple tables by specifying the links between them. And you have options to aggregate subtables.  

Queries support multiple different ways to join and flatten the tables: