Suppose sn is a CHARACTER(15) field of Serial IDs for Automobiles and positions 3 to 5 represent the country of origin as three letters.
For example:
12JAP3764-35421 37USA9873-26189 11KOR1221-13145
To search for serial IDs of cars made in the USA:
SELECT make, sn FROM autos WHERE SUBSTRING (sn FROM 3 FOR 3) = 'USA';