Question
What do the numbers refer to in GROUP BY and ORDER BY clauses?
Answer
The numbers are a way to refer to the columns in the SELECT statement (ie. 1 is the first column selected, 2 is the second column selected, etc). For example, this:
SELECT col_A, col_B
FROM tbl
ORDER BY col_A;
is equivalent to:
SELECT col_A, col_B
FROM tbl
ORDER BY 1;