How do I query the Archive programmatically?

There are two possibilities:

• scripting against standard protocols
• scripting against the ESO WDB interface

Since 2018, it is possible to access the ESO Science Archive using standard protocols and interfaces based on virtual observatory standards. In particular, the Tabular Access Protocol (TAP) can be used to perform custom queries. This also opens up the possibility to use common astronomical tools to query and access files directly from within tools like Aladin, TOPCAT, and SPLAT-VO.

Originally, queries could be sent to the ESO Science Archive programmatically using the homegrown WDB query engine. You can still use that interface: the description on how to do it is provided in a dedicated WDB page.

What is the difference between the two methods?

• WDB exposes a number of query interfaces, each defining a precise query to the database to serve specific needs. The users can enter qualifiers to constrain the results of the query, but cannot modify the underneath query. Note: WDB does not provide access to the ESO science catalogues.
  
• Through TAP, the users can gain full read-only access to the ESO databases, and can therefore write their own queries, using the SQL-based Astronomical Data Query Language (ADQL 2.0, VO standard). Note: the ESO science catalogues can be queried using a dedicated TAP interface (tap_cat).
  

How can I learn how to build my own ADQL queries, and script them for example using Python?

A dedicate and interactive page demonstrating the new functionality is available at http://archive.eso.org/programmatic/. Via that page you can learn how to formulate queries, how to script them, which URLs to use to fetch metadata, or previews, or provenance information, etc. Many examples are provided therein; you can try different precooked queries, or you can edit them and execute them directly from that page. You will find there some python scripts, and other information.