Regarding joins, you will primarily use 2: left outer join & inner join. A distant third is outer join. (BTW, the only difference between left outer & right outer are more in the head of the query's author than in any physical sense. IMHO, anyone who writes from left-to-right will probably only ever use left outer join.)
left outer join
And advice with aliases: Keep them short yet obvious. Too many people use single letters for aliases, which are OK, but can quickly lose meaning--especially between queries. Aiming for 3-5 letter aliases are still reasonable to type yet are still descriptive enough for you when you come back to a query after a long time or hand over the query to someone else. And most importantly, be consistent with your aliases! So for your employee table, don't use e one time & emp on another query. (Yes, it's allowed but it creates confusion later.)
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