Include some explanations about joins

[Gustry]

Some nice tweets :

• https://twitter.com/morsimas/status/1522878355917447168
• https://twitter.com/ageofgeeks_in/status/1523166302734471170
• https://www.linkedin.com/posts/himanshukumarmahuri_master-sql-in-16-pages-ugcPost-7092101454433304576-HE-9?utm_source=share&utm_medium=member_desktop

[Gustry]

This tweet is nice as well https://twitter.com/ValentinaCupac/status/1556906177686036480

[Gustry]

New link which is nice from @mdouchin https://towardsdatascience.com/you-should-use-this-to-visualize-sql-joins-instead-of-venn-diagrams-ede15f9583fc