The correct answer here is Costa v ENEL (1964). It established the principle of supremacy (precedence) of EU law over conflicting national legislation. More about the case here: https://en.wikipedia.org/wiki/Costa_v_ENEL

And why am I bringing this to Mastodon? Well, #Facebook groups are still a major way for people who are studying for the #EPSO to connect, share study tips, help eachother ...and do lots of complaining...In any case, why can't the Fediverse fill that space? So I plan to blog about it as I study: to promote the EU here, and to draw more EU nerds to Mastodon, who may hear that EU nerds are sharing real cool study tips over on the Fediverse... #EPSOprep