public function union(): self
    {
        $query = new self;
        $query->clauses = $this->clauses->add(
            new Clause\UnionClause
        );

        return $query;
    }
public function union(): self
    {
        $query = new self;
        $query->clauses = $this->clauses->add(
            new Clause\UnionClause
        );

        return $query;
    }
public function union ( ) : self { $ query = new self ; $ query -> clauses = $ this -> clauses -> add ( new Clause \ UnionClause ) ; return $ query ; }
Add a UNION clause

@return self
Add a UNION clause