I need help with a sql string. My table has a serial number column which is unique. The second column has a part number which can be duplicatied but not in all cases. The third column is an airplane tail number which is the same for all records in set. I need the top two serial numbers(if only 1 exists then top 1) for every distinct part number filtered by the tail number.

Example:
S/n P/n T/n
1 845-1 XYZ
2 846-1 XYZ
3 846-1 XYZ
4 846-1 XYZ
5 847-1 XYZ