public function toSql()
    {
        if (is_string($this->sql) === false) {
            $this->sql = $this->toSqlLevel($this->tree);
        }

        return $this->sql;
    }
public function toSql()
    {
        if (is_string($this->sql) === false) {
            $this->sql = $this->toSqlLevel($this->tree);
        }

        return $this->sql;
    }
Extract the corresponding SQL code.

@return string generated SQL code