DISTINCT specifies that duplicate values are not to be returned when an expression is processed.
Only one row is to be returned from any set of duplicates that might result from a given expression list. Rows are duplicates only if each value in one is equal to the corresponding value in the other.
If you specify DISTINCT and an expand_alias in the select list, the database performs the DISTINCT operation after the EXPAND operation to ensure that duplicate rows are removed from the expanded result.
If you specify DISTINCT but do not specify an expand_alias in the select list, the system ignores the EXPAND ON clause (even if you specify one), and does not expand rows.
You cannot specify DISTINCT if any member of the select column list is a LOB column.
DISTINCT is ANSI SQL:2011-compliant.