### --- 需求: 查询各个分类下的商品平均价格
~~~ # 通过 多表查询
SELECT
cname AS '分类名称',
AVG(p.`price`) AS '平均价格'
FROM products p LEFT JOIN category c ON p.`category_id` = c.`cid`
GROUP BY c.`cname`;
~~~ # 通过视图查询 可以省略连表的操作
SELECT
cname AS '分类名称',
AVG(price) AS '平均价格'
FROM products_category_view GROUP BY cname;
### --- 需求: 查询鞋服分类下最贵的商品的全部信息
~~~ 通过连表查询
~~~ 先求出鞋服分类下的最高商品价格
SELECT
MAX(price) AS maxPrice
FROM
products p LEFT JOIN category c ON p.`category_id` = c.`cid`
WHERE c.`cname` = '鞋服'
### --- 将上面的查询 作为条件使用
SELECT * FROM products p LEFT JOIN category c ON p.`category_id` = c.`cid`
WHERE c.`cname` = '鞋服' AND p.`price` =
(SELECT
MAX(price) AS maxPrice
FROM
products p LEFT JOIN category c ON p.`category_id` = c.`cid`
WHERE c.`cname` = '鞋服');
### --- 通过视图查询
SELECT * FROM products_category_view pcv
WHERE pcv.`cname` = '鞋服'
AND pcv.`price` = (SELECT MAX(price) FROM products_category_view WHERE cname =
'鞋服')
-- 使用多表方式查询
SELECT
c.`cname`,
AVG(p.`price`)
FROM products p LEFT JOIN category c ON p.`category_id` = c.`cid`
GROUP BY c.`cname`;
-- 通过视图查询
SELECT
pc.`cname`,
AVG(pc.`price`)
FROM products_category_view pc GROUP BY pc.`cname`;
-- 查询鞋服分类下最贵的商品的全部信息
-- 多表查询
-- 1.查询鞋服分类中 最高的商品价格
SELECT
MAX(p.`price`)
FROM products p LEFT JOIN category c ON p.`category_id` = c.`cid`
WHERE c.`cname` = '鞋服';
-- 2.进行子查询 将上面的查询结果作为条件
SELECT
*
FROM products p LEFT JOIN category c
ON p.`category_id` = c.`cid`
WHERE c.`cname` = '鞋服' AND p.`price` =
(
SELECT
MAX(p.`price`)
FROM products p LEFT JOIN category c ON p.`category_id` = c.`cid`
WHERE c.`cname` = '鞋服'
);
-- 通过视图查询
SELECT
*
FROM products_category_view pc
WHERE pc.`cname` = '鞋服' AND pc.`price` =
(
-- 子查询 求出鞋服分类下的最高价格
SELECT MAX(pc.`price`) FROM products_category_view pc
WHERE pc.`cname` = '鞋服' );
Walter Savage Landor:strove with none,for none was worth my strife.Nature I loved and, next to Nature, Art:I warm'd both hands before the fire of life.It sinks, and I am ready to depart ——W.S.Landor