Just learned this yesterday, so I'll post a quick aside: Dual is a dummy object with one row.
select 'foo' from dual
is equivalent to the tsql statement
select 'foo'
which is invalid in ansi sql
Dan Guzman - Not the MVP (7/22/2010)
All questions have to be prefaced by Server version and 'according to MS Docs' or 'my own personal opinion based on how much detail I felt like digging into at the time.'