MySQL：使用子查询中的join进行更新

Question

我有一张桌子上有产品和一张桌子，上面有产品评论。 产品表包含父产品和儿童产品。 母产品应该从儿童产品中获得所有评论。 我做了：

DROP TABLE IF EXISTS products;
CREATE TABLE products (
    `id` int(10) unsigned NOT NULL AUTO_INCREMENT, 
    `parent` int(10) unsigned DEFAULT NULL,
    `review` decimal(3,2) DEFAULT NULL,
    PRIMARY KEY(id)
);

DROP TABLE IF EXISTS reviews;
CREATE TABLE reviews (
    `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    `product` int(10) unsigned NOT NULL,
    `review` decimal(3,2) DEFAULT NULL,
    PRIMARY KEY(id) 
);

INSERT INTO products SET id=1, parent=null;
INSERT INTO products SET id=2, parent=1;
INSERT INTO products SET id=3, parent=1;

INSERT INTO reviews SET product=2, review=5;
INSERT INTO reviews SET product=3, review=5;
INSERT INTO reviews SET product=3, review=4;

INSERT INTO products SET id=4, parent=null;
INSERT INTO products SET id=5, parent=4;

INSERT INTO reviews SET product=5, review=4;
INSERT INTO reviews SET product=5, review=2;

UPDATE products
SET products.review=
(SELECT SUM(reviews.review)/COUNT(reviews.review) FROM reviews 
LEFT JOIN products p ON p.parent = products.id
)
WHERE products.parent IS NULL;

但有了这个我很惊讶我得到一个错误：

ERROR 1054（42S22）：'on子句'中的未知列'products.id'

有关如何正确操作的任何建议？ 这个想法是产品1应该得到14/3 = 4.66的评论，产品4应该得到6/2 = 3的评论。

Answer 1

子查询中看不到产品。 请改用以下语法：

UPDATE products pp
LEFT JOIN (
  SELECT pc.parent, SUM(r.review)/COUNT(r.review) as 'rev'
  FROM reviews r
    LEFT JOIN products pc on r.product = pc.id
  GROUP BY pc.parent
) pcc ON pcc.parent = pp.id  
SET pp.review=pcc.rev
WHERE pp.parent IS NULL;

Answer 2

由于您已将p声明为products表的别名，因此需要在整个查询中使用它。 因此，在LEFT JOIN子句中，只需使用p.parent而不是products.parent 。

UPDATE products
SET products.review=
(SELECT SUM(reviews.review)/COUNT(reviews.review) FROM reviews 
LEFT JOIN products p ON p.parent = p.id
)
WHERE products.parent IS NULL;

Answer 3

从本质上讲，您似乎在寻找这个价值：

SELECT SUM(r.review)/(SELECT COUNT(*) FROM products) n FROM reviews r;
+----------+
| n        |
+----------+
| 4.666667 |
+----------+

所以，像......

UPDATE products x 
  JOIN (SELECT SUM(r.review)/(SELECT COUNT(*) FROM products) n FROM reviews r) y
   SET x.review = y.n
 WHERE x.review IS NULL;