code: func (ssa *SQLStorageAuthority) GetCertificate(ctx context.Context, serial string) (core.Certificate, error) {
  	if !core.ValidSerial(serial) {
  		err := fmt.Errorf("Invalid certificate serial %s", serial)
  		return core.Certificate{}, err
  	}
  
  	cert, err := SelectCertificate(ssa.dbMap.WithContext(ctx), "WHERE serial = ?", serial)
  	if err == sql.ErrNoRows {
  		return core.Certificate{}, berrors.NotFoundError("certificate with serial %q not found", serial)
  	}
  	if err != nil {
  		return core.Certificate{}, err
  	}
  	return cert, err
  }
docs: // GetCertificate takes a serial number and returns the corresponding
  // certificate, or error if it does not exist.