code: public T addUniqueKey(final Connection _con,
                            final String _tableName,
                            final String _uniqueKeyName,
                            final String _columns)
          throws SQLException
      {
          final StringBuilder cmd = new StringBuilder();
          cmd.append("alter table ").append(_tableName).append(" ")
                          .append("add constraint ").append(_uniqueKeyName).append(" ")
                          .append("unique(").append(_columns).append(")");
  
          AbstractDatabase.LOG.debug("    ..SQL> " + cmd.toString());
  
          final Statement stmt = _con.createStatement();
          try {
              stmt.execute(cmd.toString());
          } finally {
              stmt.close();
          }
  
          @SuppressWarnings("unchecked") final T ret = (T) this;
          return ret;
      }
docs: Adds a new unique key to given table name.
  
  @param _con SQL connection
  @param _tableName name of table for which the unique key must be created
  @param _uniqueKeyName name of unique key
  @param _columns comma separated list of column names for which the unique
  key is created
  @return this instance
  @throws SQLException if the unique key could not be created