Using an Asterisk in Nested select Statements

You can use an asterisk (*) in a nested select statement that is not an exists subquery as long as the asterisk follows certain conditions.

The asterisk:
In addition, you can:

When an asterisk resolves to a single table column for the nested query, the query is equivalent to explicitly using a single table column.

This is a valid nested query, because t2 only has one column:
1> create table t1(c1 int, c2 int)
2> create table t2(c1 int)
1>select * from t1 where c1 in (select * from t2)
The nested select statement is equivalent to:
1> select * t1 where c1 in (select c1 from t2)
2> go