I'm curious as well it seems that if the column is in the outer Table it allows the IN (SELECT) to work

eg

CREATE TABLE #Tmp (

MyColumn Int Identity(1,1),

MyDescription varchar(100)

)

Create Table #Tmp2 (

ACol int

)

Insert into #Tmp

Values ('Hello'),('World'),('Mom')

Insert into #Tmp2

Values (1),(2),(3),(4)

Select MyColumn

from #Tmp

Where MyColumn In (Select MyColumn from #Tmp2)

Select MyColumn

from #Tmp

Where MyColumn In (Select xyz from #Tmp2)

The first select runs the second Errors as xyz isnt in either table, Can anyone explain if this is working as intended.