from table1 left outer join table2 on table1.col1 = table2.col1
group by table1.col1 having count(table2.col1) != 1
Where there is no match, table2.col1 in the outer join will be null, so the count will be zero.
Where there are multiple matches the count will be > 1.
Untested because you didn't supply table and data creation scripts. See Re: 2. How do I ask a question on the forums?