Lets talk about books again ^^ : Database System Concepts (Avi Silberschatz · Henry F.Korth · S. Sudarshan)
Here are some keywords you won't hear about in most online tutorials :
Normalization/Denormalization. When designing DB structure you will probably face a common problem : reducing redudancy of data (meaning the same data is stored only once and in one place) makes the request to get complexe views of data really complicated. So you will, at some point, have to put duplicated version of data here and there to improve performance and request readability.
Hash Table and Binary Tree : Two concept of general computing science that are used a lot in DB engines indexes to improve performance. Hash Tables will hold "numeric equivalent" of string data, which will allow the engine to look for a value with a single number (this is way faster than using a string which is one number per character at least). Binary Tree are a way of ordering numerical values to increase search performance.
You can find plenty of information about this online.