I'm looking for a diagram or schema of all Magento 2 database tables, such as Orders, Products, etc. and their relationships with each other.
Magento 2 Database Diagram – Orders, Products, and Relationships
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It's not up to date. There was a huge step in magento-1.6 and many minor changes in the other 1.x updates.
Especially the sales and index tables are completely different now, I would not use this sheet as a reference anymore.
Unfortunately table definitions are not in one place but scattered around hundreds of install and upgrade scripts. You will find them in app/code/core/Mage/*/sql/*/*.php
You will notice that there is usually a install-1.6.0.0.php script that sets up everything for the current module and Magento 1.6. Start there and ignore all scripts with lower versions. Changes are incrementally added with scripts like upgrade-1.6.0.0-1.6.1.0 and so on.
Update: Thanks to Anna Völkl, there is a new complete database diagram for Magento 1.9.2.2:
Source: http://anna.voelkl.at/magento-ce-1-9-2-2-database-diagram/
I was also curious to find an updated database diagram. Actually I could not find one so I created one myself.
Magento CE 1.9.2.2 consists of 333 tables.
This is the database diagram of a fresh CE 1.9.2.2 installation:
As I cannot upload more than 2MB here, here's the link to the full size diagram
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EDIT: Just like Magento 1 Anna Völkl created a new version for Magento 2. Check her answer.
What you are looking for is an ERD (Entity Relationship Diagram). I have searched for a Magento 2 ERD but could not find and. I think someone from the community will publish one at some point.
Here is a blog post which describes the database changes between 1.x and 2.x briefly.
So if you read that you should be able to use a 1.x ERD. Which you can find here: http://anna.voelkl.at/magento-ce-1-9-2-2-database-diagram/