code: public function addForeignKey($name, $table, $columns, $refTable, $refColumns, $delete = null, $update = null)
      {
          $sql = 'ALTER TABLE ' . $this->db->quoteTableName($table)
              . ' ADD CONSTRAINT ' . $this->db->quoteColumnName($name)
              . ' FOREIGN KEY (' . $this->buildColumns($columns) . ')'
              . ' REFERENCES ' . $this->db->quoteTableName($refTable)
              . ' (' . $this->buildColumns($refColumns) . ')';
          if ($delete !== null) {
              $sql .= ' ON DELETE ' . $delete;
          }
          if ($update !== null) {
              throw new Exception('Oracle does not support ON UPDATE clause.');
          }
  
          return $sql;
      }
docs: {@inheritdoc}