DISTINCT is not a function that applies only to some columns. It’s a query modifier that applies to all columns in the select-list.
That is, DISTINCT reduces rows only if all columns are identical to the columns of another row.
DISTINCT must follow immediately after SELECT (along with other query modifiers, like SQL_CALC_FOUND_ROWS). Then following the query modifiers, you can list columns.
-
RIGHT:
SELECT DISTINCT foo, ticket_id FROM table...Output a row for each distinct pairing of values across ticket_id and foo.
-
WRONG:
SELECT foo, DISTINCT ticket_id FROM table...If there are three distinct values of ticket_id, would this return only three rows? What if there are six distinct values of foo? Which three values of the six possible values of foo should be output?
It’s ambiguous as written.