我有一个带有
Items
表的数据库,看起来像这样:
id
name
category (int)
有几十万条记录。每个
item
可以属于 7 个不同的 categories
之一,对应于一个 categories
表:
id
category
我想要一个从每个类别中选择 1 个随机项目的查询。解决这个问题的最佳方法是什么?我知道使用
Order By rand()
和 LIMIT 1
进行类似的随机查询,但我从未做过这样的事情。
此查询返回按随机顺序加入类别的所有项目:
SELECT
c.id AS cid, c.category, i.id AS iid, i.name
FROM categories c
INNER JOIN items i ON c.id = i.category
ORDER BY RAND()
要将每个类别限制为一个,请将查询包装在 partial
GROUP BY
:
SELECT * FROM (
SELECT
c.id AS cid, c.category, i.id AS iid, i.name
FROM categories c
INNER JOIN items i ON c.id = i.category
ORDER BY RAND()
) AS shuffled_items
GROUP BY cid
请注意,当查询同时包含
GROUP BY
和 ORDER BY
子句时,将在排序之前执行分组。这就是我使用两个查询的原因:第一个查询对结果进行排序,第二个查询对结果进行分组。
我知道这个查询不会赢得任何比赛。我愿意接受建议。
这是一个简单的解决方案。假设你有这张桌子。
id name category
1 A 1
2 B 1
3 C 1
4 D 2
5 E 2
6 F 2
7 G 3
8 H 3
9 I 3
使用此查询
select
c.id,
c.category,
(select name from category where category = c.category group by id order by rand() limit 1) as CatName
from category as c
group by category
试试这个
SELECT id, name, category from Items where
(
select count(*) from Items i where i.category = Items.category
GROUP BY i.category ORDER BY rand()
) <= 1
参考:http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/
在最终选择之前更改原始表格的顺序(随机顺序):
select * from
(select category, id, name from categories order by rand()) as tab
group by 1
从mysql8开始你可以使用Windows函数解决这个问题, 避免与
category
table 的额外连接条件
https://dev.mysql.com/doc/refman/8.0/en/window-functions-usage.html
SELECT * FROM (
SELECT FIRST_VALUE(id) OVER(PARTITION BY category ORDER BY RAND()) AS first_id FROM items
Order BY RAND()) AS r
GROUP BY first_id
请注意:在下面的示例中,我假设您的表被命名为“items”而不是“Items”,因为您还说另一个表被命名为“categories”(第二个表名称未大写)。
您想要执行的操作的 SQL 大致为:
`SELECT items.id AS item_id,
items.name AS item_name,
items.category AS item_category_id,
categories.id AS category_id,
categories.category AS category_name
FROM items, category
WHERE items.category = categories.id
ORDER BY rand()
LIMIT 1`