SELECT col1, col2, col3, col4, Q1.col5, col6
FROM TABLE_NAME,
   (SELECT col5 FROM TABLE_NAME WHERE col6 = 123456 GROUP BY col5) Q1
WHERE col6 = 123456 AND TABLE_NAME.col5 = Q1.col5;


SELECT count(1), col1, col2, col3, col4, Q1.col5, col6
FROM TABLE_NAME,
   (SELECT col5 FROM TABLE_NAME WHERE col6 = 123456 GROUP BY col5) Q1
WHERE col6 = 123456 AND TABLE_NAME.col5 = Q1.col5;