别名和聚合函数的 MySQL 查询问题

Question

我有一个麻烦的 MySQL 查询如下：

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();

这个产生了“'where 子句'中的“未知列'avg_price'”错误。我理解这是因为 WHERE 子句中不允许使用列别名。 （如果我在此过程中有任何错误，请纠正我。）

所以，我像这样调整了查询​​：

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();

用实际计算替换别名，此查询产生错误：“无效使用组函数”。我理解这是因为 avg() 直到 WHERE 子句完成处理后才会发生。

然后我尝试了：

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;

将 WHERE 替换为 HAVING 并产生此错误“您的 SQL 语法有错误；请查看与您的 MySQL 服务器版本相对应的手册，以获取在 'HAVING camera_id = ir_camera_id' 附近使用的正确语法”。

在这一点上，我觉得我在黑暗中射击，试图让这个查询工作。有人会指导我正确的方向以使这个查询正常运行吗？

谢谢！

Answer 1

1. 虽然您可以使用WHERE 指定连接条件，但最好在LEFT[INNER] JOIN 子句中执行。
2. 如果要按非聚合字段过滤，请将过滤器放入WHERE，如果需要按聚合过滤，请将条件移入HAVING

3. 在同一查询中使用聚合和非聚合时，不要忘记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 子句应该在 HAVING 子句之后。 （除了你在ORDER BY rand() 后面加上一个分号; 然后继续HAVING，这实际上是另一个查询的开始，因为第一个以; 结束）。