SQL server 2005 TSQL problem

Question

Deal all\,
Table1
Table2
T1Col1
T1col2
T2Col1
T2col2
1
asdf
1
asdf
2
ghgh
2
ghgh
3
kkjk
3
kkjk
4
kkjk
4
ldflkl
5
jhjj
5
urws
Think that i have above 2 tables in SQL server 2005.
select * from table1 where T1Col1 in ( select T1Col1 from Table2)
should i except error for the above query ?
But SQL server 2005 is not giving any error , it is simple resulting all rows  from Table1
regards
ranganadh kodali

Answer 1

You will not receive any type of error because T1Col1 does exist in Table1 and even inside the parens
you are still within the context of the same query, the reason for this is that SQL Server allows you to use the values of the outer table inside the inner condition, you will find tons of examples of this usage when people use exists, here is one that's perfectly
valid:
select *
from table1
where exists (select 1 from Table2 where T1Col1 = T2Col1)
so in your particular query what's happening is that you're selecting a kind of "constant" value in your inner query, similar to doing this:
select 'constant' as myCol from Table2;
just that the 'constant' value is really coming from your outer table.
please mark as answer if it helps! :)