按组限制而不省略重复项？

Question

我有这个有点复杂的查询（精简版）：

SELECT    a.item_id,
          a.title,
          a.series,
          c.title AS manufacturer_title
FROM      catalog_items AS a
JOIN      catalog_franchises AS c ON a.manufacturer_id = c.franchise_id
JOIN      catalog_franchises AS e ON a.game_id = e.franchise_id
WHERE     e.franchise_id = 6
          AND a.valid = TRUE
          AND c.valid = TRUE
          AND e.valid = TRUE
ORDER BY  c.title, a.series, a.title

我试图关注的重要领域是c.title 和a.series。因此，为简洁起见，此查询返回：

item_id      series      manufacturer_title
46           2           fantasy flight games
63           1           gaming heads
64           1           gaming heads
33           2           reaper miniatures
124          1           triforce
125          1           triforce
45           1           triforce
43           1           usaopoly

我试图做的是添加一个基于这些字段的独特组合的LIMIT...如果我添加：GROUP BY c.title, a.series 它会给我独特的组：

item_id      series      manufacturer_title
46           2           fantasy flight games
63           1           gaming heads
33           2           reaper miniatures
124          1           triforce
43           1           usaopoly

但我想要 所有 行，受这些组的限制。所以如果限制是 3，我想要前 3 组中的所有项目：

item_id      series      manufacturer_title
46           2           fantasy flight games
63           1           gaming heads
64           1           gaming heads
33           2           reaper miniatures

我希望这是有道理的。如何修改我的查询以实现此目的？

Answer 1

您可以先在子查询中选择和限制组，然后将其连接回表以获得最终结果。

SELECT DISTINCT a2.series, c2.title
FROM catalog_items AS a2
JOIN catalog_franchises AS c2 ON a2.manufacturer_id = c2.franchise_id
JOIN catalog_franchises AS e2 ON a2.game_id = e2.franchise_id
WHERE e2.franchise_id = 6
    AND a2.valid = TRUE
    AND c2.valid = TRUE
    AND e2.valid = TRUE
ORDER BY  c2.title, a2.series
LIMIT 3

只需将其添加到原始查询的 WHERE：

AND (a.series, c.title) IN (query above)

注意：是的，is 确实会导致几乎重复自身内部的原始查询；但这通常是这类查询的最终结果。

Answer 2

修改您的原始查询以获取前 3 个组：

SELECT DISTINCT c.title, a.series
FROM      catalog_items AS a
JOIN      catalog_franchises AS c ON a.manufacturer_id = c.franchise_id
JOIN      catalog_franchises AS e ON a.game_id = e.franchise_id
WHERE     e.franchise_id = 6
          AND a.valid = TRUE
          AND c.valid = TRUE
          AND e.valid = TRUE
ORDER BY  c.title, a.series
LIMIT 3

将其用作 FROM 子句（派生表）中的子查询，以将结果限制为 3 个组：

SELECT    a.item_id,
          a.title,
          a.series,
          c.title AS manufacturer_title
FROM      catalog_items AS a
JOIN      catalog_franchises AS c ON a.manufacturer_id = c.franchise_id
JOIN      catalog_franchises AS e ON a.game_id = e.franchise_id
-- begin injected code 
JOIN (
    SELECT DISTINCT c.title, a.series
    FROM      catalog_items AS a
    JOIN      catalog_franchises AS c ON a.manufacturer_id = c.franchise_id
    JOIN      catalog_franchises AS e ON a.game_id = e.franchise_id
    WHERE     e.franchise_id = 6
              AND a.valid = TRUE
              AND c.valid = TRUE
              AND e.valid = TRUE
    ORDER BY  c.title, a.series
    LIMIT 3
) x ON a.series = x.series AND c.title = x.title
-- end injected code 
WHERE     e.franchise_id = 6
          AND a.valid = TRUE
          AND c.valid = TRUE
          AND e.valid = TRUE
ORDER BY  c.title, a.series, a.title

这与 Uueerdo 建议的基本相同，但使用 JOIN 而不是 WHERE .. IN .. 条件。

Answer 3

在单列中获取结果会有帮助吗？如果是这样，您可以使用group_concat()：

SELECT GROUP_CONCAT(a.item_id) as items,
       a.series, c.title AS manufacturer_title
FROM catalog_items a JOIN
     catalog_franchises c
     ON a.manufacturer_id = c.franchise_id JOIN
     catalog_franchises e
     ON a.game_id = e.franchise_id
WHERE e.franchise_id = 6 AND
      a.valid = TRUE AND
      c.valid = TRUE AND
      e.valid = TRUE
GROUP BY a.series, c.title
ORDER BY c.title, a.series;

虽然您可以在单独的行中获取结果，但这比在 MySQL（v8 之前）中应该更难。如果满足您的需求，这是一个相对简单的解决方案。

编辑：

一种获得你想要的东西的方法是使用变量：

SELECT ist.*
FROM (SELECT ist.*,
             (@rn := if(@st = concat_ws(':', a.series, manufacturer_title), @rn,
                        if(@st := concat_ws(':', a.series, manufacturer_title), @rn + 1, @rn + 1)
                       )
             ) as rn
      FROM (SELECT a.item_id, a.series, c.title AS manufacturer_title
            FROM catalog_items a JOIN
                 catalog_franchises c
                 ON a.manufacturer_id = c.franchise_id JOIN
                 catalog_franchises e
                 ON a.game_id = e.franchise_id
            WHERE e.franchise_id = 6 AND
                  a.valid = TRUE AND
                  c.valid = TRUE AND
                  e.valid = TRUE
            ORDER BY c.title, a.series
           ) ist CROSS JOIN
           (SELECT @rn := 0, @st := '') params
      ) ist
WHERE rn <= 3;