This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
It appears you are trying to insert a row with col_num value of 2 into the table which already contains a row with col_num value of 2, and you have defined a unique constraint over that column which this query is violating. Either remove the constraint, or make sure you are not trying to enter the same value more than once into a column which is constrained.