Union and Union All are very similar, but if you don’t understand the differences, it can cause trouble.

In this video I do a quick demonstration of the Union and Union All Function.

I use the Wide World Importers and the following scripts:

select * from [Application].[People]
where PersonID between 1 and 5
union
select * from [Application].[People]
where PersonID between 4 and 9

See the original  SQL Union and Union All video.