code: public function getSql()
      {
          if ($this->constraint instanceof ForeignKey) {
              return sprintf('ALTER TABLE %s ADD CONSTRAINT %s FOREIGN KEY (%s) REFERENCES %s (%s) MATCH SIMPLE ON UPDATE %s ON DELETE %s;'
                  , $this->constraint->getTable()->getQualifiedName()
                  , $this->constraint->getName()
                  , implode(',', $this->constraint->getColumns())
                  , $this->constraint->getReferencedTable()->getQualifiedName()
                  , implode(',', $this->constraint->getReferencedColumns())
                  , $this->constraint->getUpdateAction()
                  , $this->constraint->getDeleteAction()
              );
          }
          if ($this->constraint instanceof Unique) {
              return sprintf('ALTER TABLE %s ADD CONSTRAINT %s UNIQUE (%s);'
                  , $this->constraint->getTable()->getQualifiedName()
                  , $this->constraint->getName()
                  , implode(',', $this->constraint->getColumns())
              );
          }
      }
docs: {@inheritdoc}