[英]How can i join this 2 MySQL queries together
我有一些產品拉入的查詢,有很多產品,並且列表會不斷增長
SELECT pid, link, created_at FROM products ORDER BY timestamp DESC
但是我還有另一個表,其中包含一個pid
和rating
字段,該pid
是指向products表的鏈接。
SELECT AVG(rating) FROM ratings GROUP BY pid
我該如何查詢,以便它提取如下信息?
products.pid, products.link, products.created_at, AVG(ratings.rating)
在MS-SQL中,可以選擇以下形式:
SELECT p.pid,
p.link,
p.created_at,
(SELECT AVG(rating) FROM ratings r where r.pid = p.pid) as rating
FROM products p
ORDER BY timestamp DESC
在MySQL中,這應該給出您想要的結果。
SELECT p.pid, p.link, p.created_at, AVG(r.rating) rating
FROM products p
LEFT JOIN ratings r
ON p.pid=r.pid
GROUP BY p.pid;
附帶說明一下,MySQL是特殊的,因為它允許您執行不完整的GROUP BY
查詢,因此,如果您想在另一個數據庫上運行它(或者出於樣式原因),則查詢應該是:
SELECT p.pid, p.link, p.created_at, AVG(r.rating) rating
FROM products p
LEFT JOIN ratings r
ON p.pid=r.pid
GROUP BY p.pid, p.link, p.created_at;
SELECT p.pid, link, created_at, avg_rating
FROM products p
JOIN (SELECT pid, AVG(rating) avg_rating
FROM ratings
GROUP BY pid) a
ON p.pid = a.pid
ORDER BY timestamp DESC
當我使用上面的其他帖子時,它總是向我顯示此錯誤:
'Unknown column "timestamp" in "order clause"'
我相信時間戳會添加到產品表的created_at列中。 因此,我在此列本身上使用了排序。 它將從產品表中選擇最后一行,並隨其一起打印AVG(rating)。
SELECT products.pid, products.link, products.created_at, AVG(ratings.rating)
FROM products, ratings
WHERE products.pid = ratings.pid
GROUP BY products.pid
ORDER BY products.created_at ASC
;
此查詢已經過測試,對我來說工作正常。
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