MySQL查詢問題的別名和聚合函數

Question

我有一個麻煩的MySQL查詢如下：

SELECT camera_id, ((avg(low_price) + avg(high_price))/2) as avg_price 
FROM camera_general, camera_products 
WHERE camera_id = ir_camera_id 
AND dp_post_dt IS NOT NULL 
AND dp_post_dt NOT LIKE '0000%' 
AND currently_manufactured = 'Yes' 
AND ((no_of_sellers >= 0) OR ((TO_DAYS(CURRENT_DATE) - TO_DAYS(dp_post_dt)) < 120)) 
AND avg_price < 150 
AND camera_id != 1411 
AND camera_id != 9 
ORDER BY rand(); 

這一個「在‘where子句’未知列'avg_price」錯誤產生的。我明白這是因爲在WHERE子句中不允許使用列別名。 （糾正我，如果我錯了，任何的這個，我去請。）

所以，我調整了像這樣的查詢：

SELECT camera_id, ((avg(low_price) + avg(high_price))/2) as avg_price 
FROM camera_general, camera_products 
WHERE camera_id = ir_camera_id 
AND dp_post_dt IS NOT NULL 
AND dp_post_dt NOT LIKE '0000%' 
AND currently_manufactured = 'Yes' 
AND ((no_of_sellers >= 0) OR ((TO_DAYS(CURRENT_DATE) - TO_DAYS(dp_post_dt)) < 120)) 
AND ((avg(low_price) + avg(high_price))/2) < 150 
AND camera_id != 1411 
AND camera_id != 9 
ORDER BY rand(); 

與實際計算更換別名，這個查詢產生錯誤：「組函數的使用無效」。我明白這是因爲在WHERE子句完成處理之後才能發生avg（）。

於是我嘗試：

SELECT camera_id, ((avg(low_price) + avg(high_price))/2) as avg_price 
FROM camera_general, camera_products 
ORDER BY rand(); 
HAVING camera_id = ir_camera_id 
AND dp_post_dt IS NOT NULL 
AND dp_post_dt NOT LIKE '0000%' 
AND currently_manufactured = 'Yes' 
AND ((no_of_sellers >= 0) OR ((TO_DAYS(CURRENT_DATE) - TO_DAYS(dp_post_dt)) < 120)) 
AND avg_price < 150 
AND camera_id != 1411 
AND camera_id != 9; 

與HAVING更換WHERE和它產生這個錯誤「您的SQL語法錯誤;檢查對應於您的MySQL服務器版本的權利手冊在'HAVING camera_id = ir_camera_id'附近使用的語法「。

在這一點上，我覺得我在黑暗中拍攝試圖使這個查詢工作。有人會指引我在正確的方向，使這是一個功能正常的查詢？

謝謝！

Answer 1

1. 即使你可以用WHERE指定連接條件，最好在LEFT[INNER] JOIN條款做。
2. 如果你想通過非集合場過濾，把過濾器進入WHERE，如果你需要通過聚集來過濾，並移動條件爲HAVING

3. 雖然使用聚合非聚集在同一查詢，不要」忘記GROUP BY。

   SELECT camera_id, ((avg(low_price) + avg(high_price))/2) as avg_price FROM camera_general
INNER JOIN camera_products ON (camera_id = ir_camera_id)
WHERE dp_post_dt IS NOT NULL
AND dp_post_dt NOT LIKE '0000%'
AND currently_manufactured = 'Yes'
AND ((no_of_sellers >= 0) OR ((TO_DAYS(CURRENT_DATE) - TO_DAYS(dp_post_dt)) < 120))
AND camera_id != 1411 AND camera_id != 9
GROUP BY camera_id
HAVING avg_price < 150
ORDER BY rand();

Answer 2

ORDER條款應去HAVING條款。 （除了你在ORDER BY rand()之後加上分號;，然後繼續HAVING，這實際上是另一個查詢的開始，因爲第一個以;結束）。