Since my duties as web developer also require me to be my own database administrator, I’ve learned the hard way how important databases are to web technologies. I’ve been a SQL developer as much as a web developer my entire career.
I’ve worked with MSSQL, Oracle, MySQL, and now MariaDB at varying points in my career, but truly feel comfortable with MySQL/MariaDB since it pairs so well with my other development strengths. A list of related technologies probably isn’t relevant to my SQL skillset, but I’m very comfortable with normalization, triggers, stored procedures, views, and complicated joins and selects.