MySQL：AVG 函数：为什么查询返回不正确的值？

Question

所以我运行了这个小查询，似乎发生了一些奇怪的事情。

我想显示所有库存大于平均库存的商品（如果有意义的话...）。

当我运行它时，其中一种产品的库存量低于计算的平均值……我不知道为什么。在这种情况下，不应该返回的项目是：421

返回数据：

"sum(quantity_stocked)" "count(product_id)" "avg(quantity_stocked)"
"15885"                 "18"                "882.5000"


"product_id"    "quantity_stocked" 
"861"           "1082" 
"591"           "1948" 
"485"           "1402"
"421"           "858" 
"277"           "1920" 
"214"           "902" 
"59"            "1021"

查询：

select sum(quantity_stocked), count(product_id), avg(quantity_stocked) 
FROM products;

SELECT product_id, quantity_stocked
FROM products
WHERE quantity_stocked > (
    SELECT avg(quantity_stocked)
    FROM products
    GROUP BY product_id
    LIMIT 1
    );

剩余代码：

CREATE TABLE products 
(
product_ID              int(10)             UNIQUE,
product_name            varchar(30)         NOT NULL,           
quantity_stocked        int(5),
product_cost            dec(8,2)            NOT NULL,
PRIMARY KEY (product_ID)
);

INSERT INTO products VALUES
(452,'Nike SS Pro Top',418, 22.99),
(861,'Nike LunarGlide 7',1082, 74.99),
(453,'Nike LS Pro Top',654, 64.99),
(454,'Nike SS Pro Top',720, 64.99),
(451,'Nike Pro Tank',480, 45.99),
(485,'Nike 5k Run Short',1402, 24.99),
(582,'Nike Fit Capri',750, 29.99),
(591,'Nike Be Fast Tight',1948, 39.99),
(847,'Nike Legend Tee',120, 34.99),
(277,'Nike Miller v-neck',1920, 44.99),
(135,'Adidas Soccer Pant',630, 34.99),
(456,'Adidas SS Breathe Top',754, 19.99),
(457,'Adidas Pro Cleat',626, 44.99),
(214,'Adidas Slide',902, 19.99),
(054,'Underarmor SL Top',854,54.99),
(059,'Undersarmor LS High Neck',1021,54.99),
(574,'Underarmor TR Short',746,24.99),
(421,'Underarmor TR Balance Shoe',858,84.99);

Answer 1

在您的子查询中，您将比较 一个 产品的平均值（在此特定示例中，将是 #452，quantity_stocked 为 418），而不是 的平均值>所有产品。只需删除这部分：

GROUP BY product_id
LIMIT 1