相关子查询和 GROUP BY

Question

我开始学习 Mysql，但在理解 相关子查询 时遇到问题，我搜索了定义说，它每行执行一次，但是当涉及到示例时：

DEPOT (N_Dpt, Name_Dpt, City)
STORE (N_Dpt #, N_Product #, Qt_Stored)
PRODUCT (N_Product, Label_Product, Price_U, Weight, Color)  


               

每种产品类型的最高单价列表

SELECT Product_Label, Price_U FROM Product AS P1  
WHERE Price = (SELECT MAX (Price_U) FROM PRODUCT P2  
WHERE P2. Product_Label = P1. Product Label )

第一个问题：为什么它是相关子查询？
第二个问题：为什么我们不能使用 GROUP BY？

SELECT Product_label, MAX(Price_U) FROM PRODUCT GROUP BY Product_label

Answer 1

考虑以下...

DROP TABLE IF EXISTS product;

CREATE TABLE product
(id SERIAL PRIMARY KEY
,product_label CHAR(1) NOT NULL
,price_u INT NOT NULL
);

INSERT INTO product VALUES
(1,'a',12),
(2,'a',18),
(3,'a',4),
(4,'b',9),
(5,'b',1),
(6,'c',13);


-- Correlated subquery:

SELECT product_label
     , price_u
  FROM product p1  
 WHERE price_u = (SELECT MAX(price_u) FROM product p2 WHERE p2.product_label = p1.product_label);
                                                      -----------------------------------------

 +---------------+---------+
 | product_label | price_u |
 +---------------+---------+
 | a             |      18 |
 | b             |       9 |
 | c             |      13 |
 +---------------+---------+
 
-- Uncorrelated subquery

SELECT x.product_label
     , x.price_u 
  FROM product x
  JOIN 
     ( SELECT product_label
            , MAX(price_u) price_u
         FROM product 
        GROUP
           BY product_label
     ) y
    ON y.product_label = x.product_label
   AND y.price_u = x.price_u;

 +---------------+---------+
 | product_label | price_u |
 +---------------+---------+
 | a             |      18 |
 | b             |       9 |
 | c             |      13 |
 +---------------+---------+
   
-- No subquery

SELECT x.product_label
     , x.price_u 
  FROM product x
  LEFT
  JOIN product y
    ON y.product_label = x.product_label
   AND y.price_u > x.price_u
 WHERE y.id IS NULL;

 +---------------+---------+
 | product_label | price_u |
 +---------------+---------+
 | a             |      18 |
 | b             |       9 |
 | c             |      13 |
 +---------------+---------+

以上内容并不详尽