If you just the rows in which :Accident Number starts with CH you can use Starts With()
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Aircraft Incidents.jmp");
dt << Select Where(Starts With(:Accident Number, "CH"));
And with subset I would use Get Rows Where
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Aircraft Incidents.jmp");
rows_of_interest = dt << Get Rows Where(Starts With(:Accident Number, "CH"));
dt_ch = dt << Subset(Rows(rows_of_interest), Selected Columns(0), Output Table("CH Accidents"));
-Jarmo