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Separate names with a comma.
Your best bet would be to look into Regular Expressions (regex)
Something for flavor
pst = ConnectionPool.getConnection().prepareStatement("SELECT rank FROM player_database WHERE p_name = '" + p_name + "'");
Don't you need to set a specific UUID for it to have "Harold" 's skin?
Try adding debug messages. Maybe you're building the plugin in a different directory that's not in your actual plugins folder?