Your knowledge is yours; that's what lets you get hired by another company while making use of skills developed at your current company, and that's what lets you talk about your previous work at interviews. Your employment agreement probably carves out exceptions for things like trade secrets and customer lists, but not for just understanding MySQL.

Creative expressions are subject to copyright. Ideas, facts, and knowledge are not. Therefore, any actual writing or code samples you do at work are likely owned by your employer, but things you have learned are not.

I recently learned some interesting quirks of the select system call at work and wrote up a two-page document about it for internal circulation. I'll likely write up a personal blog post also, but on my own time without reference to the text I wrote at work. (The text will have to be different anyway, because it won't reference the specific OS and kernel versions we have at work, nor the specific recommendations I had for what to do about it.)

Your employment agreement might have more to say about this.