|NO.Z.00063|——————————|BigDataEnd|——|Java&MySQL索引/存储过程/触发器.V04|——|MySQL.v04|使用视图|进行查询操作|

一、使用视图进行查询操作

### --- 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')

二、sql语句

### --- sql语句
~~~     使用视图进行查询操作

-- 查询各个分类下的商品平均价格
/*
    1.查询哪些表 分类表 商品表
    2.查询条件是什么  分组操作
    3.要查询的字段    平均价格,分类名
    4.多表的连接条件 category_id = cid
*/

-- 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' ); 

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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

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