Assuming it is relatively small and unsorted by default:

#!/usr/bin/perl -w
my $db = '/some/file';
my $letter = 'Y';
open DB, $db or die "Can't read $db: $!\n";
my @matches;
while(<DB>){
    push @matches, $_ if m/^$letter/i;
}
close DB;
print sort @matches;
[download]

Or more succinctly on *nix:

$ grep -i -e ^Y db.file | sort
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