Union or UNION ALL

You can merge with union or union all when two or more separate queries need to be merged together.

The union will return all rows that have been stripped of duplicates, and union all will return all rows. At the same time, the union all consumes less resources than union, and the union removes duplicate rows from the query, and Oracle removes the duplicate rows by sorting before oracle10g, and after oracle10g uses the hash union to remove the duplicate rows, if we want the result set to be sorted, You must have the order BY statement displayed.

This article is from the "DBA Sky" blog, so be sure to keep this source http://kevinora.blog.51cto.com/9406404/1672917

Union or UNION ALL