Question

下面的选择查询返回1行，应该是3.我很确定这是因为AVG（k.sumtotal）字段。

如果我重写查询并取出该AVG（k.sumtotal）列并取出FROM inv_ratings AS k，我得到我的3行。

我无法弄清楚为什么这个表（inv_ratings）或这个AVG（k.sumtotal）列将行数限制为1.我在网上看了几个小时试图找到有关使用AVG子句返回结果的信息并且没有找不到多少。我是否必须使用group by子句，我尝试过，只会出错。

$p = "SELECT i.invention_id, i.inv_title, i.date_submitted, i.category_id,"
    . " i.approved, c.category_id, c.category, u.image_name, AVG(k.sumtotal)"
    . " FROM inv_ratings AS k INNER JOIN inventions AS i USING (invention_id)"
    . " INNER JOIN categories AS c USING (category_id)"
    . " INNER JOIN images AS u USING (invention_id)"
    . " WHERE c.category_id = $cat AND i.approved = 'approved'"
    . " HAVING u.image_name < 2"
    . " ORDER BY date_submitted"
    . " DESC LIMIT $start, $display";

$q = mysqli_query($dbc, $p) or trigger_error("Query: $p\n<br />mysqli Error: " . mysqli_error($dbc));

Answer 1

你遇到了MySQL的一个陷阱：

http://dev.mysql.com/doc/refman/5.5/en/group-by-handling.html

我讨厌MySQL曾经允许这种语法，因为它只会引起混淆。但你可能想要的是（使用MySQL的hackish行为，请注意，如果除engine_id或sumtotal之外的任何字段有多个值，则从该列获得随机值）：

$p = "SELECT i.invention_id, i.inv_title, i.date_submitted, i.category_id,"
. " i.approved, c.category_id, c.category, u.image_name, AVG(k.sumtotal)"
. " FROM inv_ratings AS k INNER JOIN inventions AS i USING (invention_id)"
. " INNER JOIN categories AS c USING (category_id)"
. " INNER JOIN images AS u USING (invention_id)"
. " WHERE c.category_id = $cat AND i.approved = 'approved'"
. " GROUP BY i.invention_id "
. " HAVING u.image_name < 2"
. " ORDER BY date_submitted"
. " DESC LIMIT $start, $display";

或者，不要使用MySQL的hackish行为：

$p = "SELECT i.invention_id, i.inv_title, i.date_submitted, i.category_id,"
. " i.approved, c.category_id, c.category, u.image_name, AVG(k.sumtotal)"
. " FROM inv_ratings AS k INNER JOIN inventions AS i USING (invention_id)"
. " INNER JOIN categories AS c USING (category_id)"
. " INNER JOIN images AS u USING (invention_id)"
. " WHERE c.category_id = $cat AND i.approved = 'approved'"
. " GROUP BY i.invention_id, i.inv_title, i.date_submitted, i.category_id,"
. " i.approved, c.category_id, c.category, u.image_name "
. " HAVING u.image_name < 2"
. " ORDER BY date_submitted"
. " DESC LIMIT $start, $display";

由于AVG，选择返回1个结果而不是3个结果

1 个答案: