When you’re learning SQL DML, the most complicated clause is typically the GROUP BY. It’s a fairly simple grouping based on values from the FROM clause in a SELECT statement. It’s what a mathematician would call an equivalence relation. This means that each grouping, or equivalence class, has the same value for a given characteristic function and the rows are all partitioned into disjoint subsets (groups) that had the same value under that function. The results table can also have columns that give group characteristics; I will get into those in a minute.
In the case of the usual GROUP