I call bullshit, do not listen to such advice, grab a SQL book, install a SQL database management server (postgres for example) and start playing with it. Create tables, do left joins, learn about indexes, try playing with pivots, and some more advanced stuff like, try building a "data warehouse" type of structure. I can guarantee you, from personal experience, it is worth it. Bonus points if you do all that from the terminal, using psql for example. It makes you feel more confident when debugging a real issue and really impresses in interviews.