outer join - merge (x= df1, y=df2, by= “id”, all= TRUE)
left join - merge (x= df1, y= df2, by = “id”, all.x = TRUE)
right join - merge (x= df1, y= df2, by = “id”, all.y = TRUE)
inner join - merge (x= df1, y= df2, by = “id”)
cross join - merge (x= df1, y= df2, by = NULL)
