Question

我正在尝试创建一个查询来列出特定类型图书的平均价格，但前提是平均价格高于500.

这是我目前的查询：

SELECT TYPE,
       IF(Avg(pages) > 500, Round(Avg(pages)), NULL) AS "Average Pages"
FROM   titles
GROUP  BY TYPE;

这给了我这个：

+------------+---------------+
| type       | Average Pages |
+------------+---------------+
| biography  |          NULL |
| children   |          NULL |
| computer   |          1226 |
| history    |          NULL |
| psychology |           512 |
+------------+---------------+

这当然包括NULL值，当我试图实现这个时：

+------------+---------------+
| type       | Average Pages |
+------------+---------------+
| computer   |          1226 |
| psychology |           512 |
+------------+---------------+

虽然我不确定这样做的最好方法。我已经搞乱了WHERE语句，但没有运气。

Answer 1

对于聚合函数，您可以使用

过滤结果

  $row = $this->baseClass->db->pdoQuery('SELECT SUM(user_have_group.user_group = 2 ) AS boosters , count(user.id ) AS created_accounts FROM `user` LEFT JOIN `user_have_group` ON user_have_group.user_id = user.id ')->result();

Answer 2

你走了：

SELECT type, AVG(pages) AS "Average Pages"
FROM titles
GROUP BY type
HAVING AVG(pages) >= 500;

Answer 3

您是否曾尝试IS NOT NULL这样：

SELECT 
    `type`,
    IF(Avg(`pages`) > 500, Round(Avg(`pages`)), NULL) AS `avg_pages`
FROM `titles`
WHERE `avg_pages` IS NOT NULL
GROUP BY `type`;

如何使用聚合查询仅显示非空值？

3 个答案: