为什么此查询在MySQL 5.1.56中失败？

Question

The following query fails in MySQL 5.1.56:

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

The error is

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

This modification without the user table does not fail:

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

Neither query fails in MySQL 5.0.67. (I exported the database from 5.0.67 and imported into 5.1.56 so the structure should be identical.)

The products table does have a product_id column, of type int(10). The favorites table also has a product_id column of type int(10). What is going on?

Answer 1

As easy as swap from tables order:

FROM users, products LEFT JOIN

Be careful, you are mixing join notations .

Answer 2

JOIN processing operator precedence has changed in MySQL in 5.1. It's a common problem for people upgrading from 5.0 MySQL LEFT JOIN after 5.0.12 changes - How to rewrite query

Answer 3

This is your original query, reformatted a little and with two parentheses added:

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

The parentheses indicate how the SQL parser is interpreting the query, and there is no products table within the added parenthesis.

It is a bad idea to mix the old style and new (as in, since SQL-92) style joins.

Use:

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

The != join is going to be slow (it practically a Cartesian product).