I'm a frontend web developer, but for a year I worked on a custom ETL system and wrote lots & lots of SQL. I still find myself using knowledge I learned back t

I'm a frontend web developer, but for a year I worked on a custom ETL system and wrote lots & lots of SQL. I still find myself using knowledge I learned back then and applying it today - for example, being able to review BI's queries is immensely useful. I'm also much less intimidated when I look at backend code, since a lot of it is interfacing with a database in some way. I think SQL is one of those essential "secondary" skills for developers.