从一个表中获取多行到另一个表的每一行

Question

我这里有一个问题，我想知道是否可以只在一个查询中执行此操作，而不是让服务器执行多个查询。

所以事情就是这样，我想要的是，在每个类别中，在 24 家商店的限制内搜索属于该类别的所有商店。也就是说，假设我注册了 5 个类别，我将搜索属于该类别的 24 家商店，得出的结论是，我将在查询结果中总共有 120 家商店。

以下代码是一个示例，但这只会获取任何类别的前 24 个商店，而不会遍历每个类别。

SELECT *
FROM categories c
    LEFT JOIN (SELECT * FROM stores_categories LIMIT 24 OFFSET 0) sc ON sc.id_category = c.id
WHERE type = 'STORE' OR type = 'ALL';

有人给我发了这个，它与我的问题非常相似，How to SELECT the newest four items per category?，有点像那样，但我希望能够限制和偏移来制作页面，而不仅仅是前 24 个最近的在每个类别中。

我得到的链接中的示例代码：

SELECT sc1.*
FROM stores_categories sc1
    LEFT OUTER JOIN stores_categories sc2 ON (sc1.id_category = sc2.id_category AND sc1.id_store < sc2.id_store)
GROUP BY sc1.id_store
HAVING COUNT(*) < 24
ORDER BY sc1.id_category;

我的数据库服务器是5.5.64-MariaDB版本。一个小提琴示例，可以使用我的数据库中的相同值进行测试。 https://www.db-fiddle.com/f/jcowJL9S4FQXqKg2yMa8kf/0

Answer 1

它可以使用每个类别的计算 row_number。

--
-- Emulated row_number via variables
-- 
SELECT sc.id, id_store
, cat.name AS cat_name
, cat.type AS cat_type
, rn
FROM
(
  SELECT sc.*
  , @rn := CASE 
           WHEN @cat_id = sc.id_category
           THEN @rn + 1 ELSE 1
           END AS rn
  , @cat_id := sc.id_category as cat_id
  FROM stores_categories sc
  CROSS JOIN (SELECT @cat_id:=0, @rn:=0) vars
  ORDER BY sc.id_category, sc.id_store DESC, sc.id DESC
) sc
LEFT JOIN categories cat 
  ON cat.id = sc.id_category
WHERE rn BETWEEN 1 AND 24
ORDER BY sc.id_category, sc.id_store DESC, sc.id DESC

或者

--
-- Grouped self-join
-- 
SELECT sc.id, sc.id_store
, cat.name AS cat_name
, cat.type AS cat_type
, COUNT(sc2.id_store) AS cnt
FROM stores_categories sc
LEFT JOIN stores_categories sc2
  ON sc2.id_category = sc.id_category 
 AND sc2.id_store >= sc.id_store
LEFT JOIN categories cat 
  ON cat.id = sc.id_category
GROUP BY sc.id
HAVING cnt BETWEEN 1 AND 24
ORDER BY sc.id_category, sc.id_store DESC;

或者

--
-- Correlated sub-query
-- 
SELECT sc.id, id_store
, cat.name AS cat_name
, cat.type AS cat_type
, ( select count(*)
    from stores_categories sc2
    where sc2.id_category = sc.id_category 
      and sc2.id_store >= sc.id_store
  ) as rn
FROM stores_categories AS sc
LEFT JOIN categories cat 
  ON cat.id = sc.id_category
HAVING rn BETWEEN 1 AND 24
ORDER BY sc.id_category, sc.id_store DESC;

dbfiddle here

上的演示