为什么此查询在MySQL 5.1.56中失败？

Question

以下查询在MySQL 5.1.56中失败：

SELECT 
shop_id, products.product_id AS
product_id, brand, title, price, image, image_width, image_height

FROM products, users LEFT JOIN

(
    SELECT fav5.product_id AS product_id, SUM(CASE 
    WHEN fav5.current = 1 AND fav5.closeted = 1 THEN 1
    WHEN fav5.current = 1 AND fav5.closeted = 0 THEN -1
    ELSE 0
    END) AS favorites_count
    FROM favorites fav5
    GROUP BY fav5.product_id 

) AS fav6 ON products.product_id=fav6.product_id
WHERE products.product_id= 46876 AND users.user_id!=products.product_id

错误是

#1054 - Unknown column 'products.product_id' in 'on clause'

没有用户表的修改不会失败：

SELECT 
shop_id, products.product_id AS
product_id, brand, title, price, image, image_width, image_height

FROM products LEFT JOIN

(
    SELECT fav5.product_id AS product_id, SUM(CASE 
    WHEN fav5.current = 1 AND fav5.closeted = 1 THEN 1
    WHEN fav5.current = 1 AND fav5.closeted = 0 THEN -1
    ELSE 0
    END) AS favorites_count
    FROM favorites fav5
    GROUP BY fav5.product_id 

) AS fav6 ON products.product_id=fav6.product_id
WHERE products.product_id= 46876

在MySQL 5.0.67中，查询均不会失败。 （我从5.0.67导出数据库，然后导入5.1.56，因此结构应相同。）

产品表确实有一个int（10）类型的product_id列。 收藏夹表还具有类型为int（10）的product_id列。 到底是怎么回事？

Answer 1

与从表顺序交换一样容易：

FROM users, products LEFT JOIN

请注意，您正在混合联接符号 。

Answer 2

在MySQL中，JOIN处理运算符的优先级在5.1中已更改。 在5.0.12更改后 ，人们从5.0 MySQL LEFT JOIN升级是一个常见问题-如何重写查询

Answer 3

这是您的原始查询，对其进行了重新格式化，并添加了两个括号：

SELECT shop_id, products.product_id AS
       product_id, brand, title, price, image, image_width, image_height
  FROM products, 
       (  -- Parenthesis added
       users LEFT JOIN
       (
            SELECT fav5.product_id AS product_id, SUM(CASE 
            WHEN fav5.current = 1 AND fav5.closeted = 1 THEN 1
            WHEN fav5.current = 1 AND fav5.closeted = 0 THEN -1
            ELSE 0
            END) AS favorites_count
            FROM favorites fav5
            GROUP BY fav5.product_id 
        ) AS fav6 ON products.product_id=fav6.product_id
    )  -- Parenthesis added
WHERE products.product_id= 46876 AND users.user_id!=products.product_id

括号表示SQL解析器如何解释查询，并且括号中没有products表。

混合使用旧样式和新样式（例如，自SQL-92起）是一个坏主意。

采用：

SELECT shop_id, products.product_id AS
       product_id, brand, title, price, image, image_width, image_height
  FROM products JOIN users ON users.user_id != products.product_id
  LEFT JOIN
       (SELECT fav5.product_id AS product_id,
               SUM(CASE WHEN fav5.current = 1 AND fav5.closeted = 1 THEN 1
                        WHEN fav5.current = 1 AND fav5.closeted = 0 THEN -1
                        ELSE 0
                   END) AS favorites_count
          FROM favorites fav5
         GROUP BY fav5.product_id 
       ) AS fav6 ON products.product_id=fav6.product_id
 WHERE products.product_id = 46876

!=连接将会很慢（实际上是笛卡尔积）。