ELI5: what is a left and right join

What are LEFT and RIGHT joins?

A LEFT join is like inviting all your friends from the left side of the room to a party, and then checking if anyone from the right side matches with them [2][3]. A RIGHT join does the opposite—it invites everyone from the right side and checks if anyone from the left matches [2]. It's the same result, just depends which group you invite first [1].

How do they work?

Imagine you have a list of students on the left and a list of test scores on the right. A LEFT join keeps all the students, even if some don't have test scores [2]. A RIGHT join keeps all the test scores, even if some students are missing [2].

When do you use each one?

You'd use a LEFT join when you care most about the left table's data and want to see if it matches anything on the right [2]. You'd use a RIGHT join when the right table is more important [4]. But really, you can usually just flip which table comes first and use LEFT join instead [1][4].

Why does the order matter?

When you write SQL, the first table mentioned is the "left" one and the second is the "right" one [3]. So which join you pick depends on which table you listed first and which data you want to keep [1][3].