找到价格最高和最低的信息：
mysql> SELECT @min_price:=MIN(price),@max_price:=MAX(price) FROM shop;
mysql> SELECT * FROM shop WHERE [email protected]_price OR [email protected]_price;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0003 | D | 1.25 |
| 0004 | D | 19.95 |
+---------+--------+-------+