What is SQL/ADQL?
SQL (Structured Query Language) is a programming language which allows you to access, filter and process data when stored in a database. ADQL (Astronomical Data Query Language) builds on top of SQL, adding additional astronomy-specific features to the language. There are a large number of tutorials and resources online about learning SQL, such as:
There are fewer ADQL tutorials online, but the GAIA team produced a tutorial, which also references other online resources for learning ADQL.
Additionally, you can download TOPCAT, which can be a bit easier to get started with, which can help write SQL/ADQL queries. The TOPCAT website also includes links to more resources on ADQL.