Looking for free or cheap places to learn some SQL and XML

This is the only answer you need: Codecademy, I have helped teach so many people (and myself!) through that service.

Seriously, their SQL class (and HTML/Python/etc) are some of the best you could ever pay for, and it's free.

For XML, not really needed these days, but if you know HTML, then you know XML, and if you don't know HTML, then check out the HTML track on that site and you'll know it!

I'd like to add on that with something like XML as people have mentioned that while it is very trivial to understand the format, I have found it not very trivial to actually interact with XML. The different formats for data like XML, JSON, YAML are just really rules for parsing a text file. In the end you are almost going to be more concerned with how the programming language you are using will be reading them in.

Granted, I haven't had to interact with XML in a few years so may be dated in saying this, but found it always a bigger job than I wanted it to be to do things with an XML file. So I could easily create one for a small example, but the actual act of parsing it into a data structure and doing any basic CRUD operations on it always took a little longer than it would seem at first glance.

More recently, I tend to see JSON or YAML in place of where I used to see XML, and it seems to me a lot of json libraries have been easier to work with, but generally I've only used JSON to quickly serialize/deserialize things rather than reading in larger sets of data with it. I do overall feel that XML is on the way out and it may be worth focusing on JSON and/or YAML over it.

SQL can be a pretty big topic on its own and you would really benefit from something more structured than only tutorials and project learning. You can get the initial stuff out of the way by tutorialing though, it helps the formal class have context if you are a bit aware of what is going on ahead of time.

Once you've got a basic grasp of SQL, you can play The SQL Murder Mystery.

https://www.sqlzoo.net/wiki/SQL_Tutorial is another good resource. Very easy to get into and has a very natural progression.

In addition to the great resources everyone else has shared, if you're looking for somewhere to practice or test (beyond something like codecademy interactive exercises), you can install a free version of Microsoft SQL on your local computer - it's the developer version on their downloads page.

Then install SQL Server Management Studio (SSMS) on your local computer as well, and you can connect the developer server and create databases, tables, views, stored procedures, etc exactly as you would in an enterprise (Windows) environment.

If you want some test data, Microsoft has a set they call AdventureWorks.

And keep in mind all of this information is assuming your (current or future) employer is using MS SQL. There are some variations between MS SQL, MySQL, and Oracle's implementation, but focusing on learning one will get you far enough to work with any of them (perhaps with a translation cheat sheet by your side).

I can only really learn by doing something, so my advice is to start doing it. You quite possibly have SQLite already installed on your computer. If not, go and download it. Find a sample dataset and start doing something. When you can’t figure out how to do a query, look it up and learn.

I haven’t done anything with xml though.

XML is just a very basic markup language. If you know HTML, you basically already know XML. Most use cases don’t actually require you to even know much about it because there are tools that will write it for you. That being said, XML by itself is useless; it really only matters in the context in which it is used, so if there is something motivating you to learn about it it might be a better idea to dive into that rather than XML itself.

SQL is harder, but not nearly as hard as many would have you believe. I would honestly recommend just searching your local library for a book about it because the thing that people tend to stumble on is that they don’t understand the ideas that inform it. You might be surprised to see you already know most of it, with the hardest stuff being the terms used to describe them. Personally, the thing I find hardest to remember in the off times I have to write an SQL query is the correct way to pass strings (it’s just different enough from everything else that I forget every time) and the way it orders operations.

One thing to be aware of is that there is essentially no real standard for SQL. There is a “base”, but every database has its own rules, caveats, and extensions. It’s better if you choose a book that covers a specific database, but to make note when it mentions that a feature is specific to it. Might I recommend Postgres? Though if you intend to learn for professional reasons it might be better to learn MS SQL or Oracle.

https://sqlbolt.com/ is a nice free way of learning simple SQL

I'd say other commentd are mostly right about XML just being like HTML. Basic XML is perhaps deceptively straightforward –– just nested arbitrary tags. Places where it gets weird are:

• schema – rules about structure allowed in an XML file
• namespaces – related to schema and allowing multiple schema with possibly colliding tag names to coexist
• parsing – stream parsing, which us event based and potentially works on very large files vs DOM parsing there you have the whole data set in memory like you would with HTML
• whitespace handling – For example, parsing an XML file and then rewriting it may not produce exactly the same file, and it may destroy information depending on the data. Different parsing libraries may have special data structures to preserve leading and trailing whitespace.
• CDATA structure for encoding binary data

Not that you necessarily need all this to do XML, but if you're looking to learn, there be the dragons.

On the SQL side, a graphical tool like DBeaver is very useful for building database schema, and even testing queries and seeing the results. I learned a lot by building up schema and seeing all the bits it added (like encoding, null/not null, etc) when you dump the schema to SQL.

I'd say a good exercise might be to try to find a pretty complicated existing XML dataset to work with, develop a database schema for it, and parse it out of XML and get it into the database with SQL.