Nulls Position

The keyword NULLS defines where NULL in the associated column will fall in the sort order: NULLS FIRST places the rows with the NULL column above rows ordered by that column’s value;NULLS LAST places those rows after the ordered rows.

NULLS FIRST is the default.

Not-parenthesized query expressions contributing to a UNION cannot take an ORDER BY clause.You can order the entire output, using one ORDER BY clause at the end of the overall query, or use parenthesized query expressions, which do allow ORDER BY.

The simplest — and, in some cases, the only — method for specifying the sort order is by the ordinal column position.However, it is also valid to use the column names or aliases, from the first contributing query only.

The ASC/DESC and/or NULLS directives are available for this global set.

If discrete ordering within the contributing set is required, use parenthesized query expressions, derived tables, or common table expressions for those sets.