SQL select rows with only a certain value in them

Question

I have a table as such

Col 1 Col 2 Col 3
1       A     1
2       A     2
3       B     1
4       C     1
5       C     2
6       D     1

Ho

Answer 1

SELECT col2
FROM your_table
GROUP BY col2
HAVING MAX(col3) = 1 AND MIN(Col3) = 1

Or

SELECT a.col2 
FROM your_table a
WHERE a.col3=1 AND NOT EXISTS(SELECT *
                              FROM your_table b 
                              WHERE a.col2=b.col2 AND b.col3<>1)