mysql - 一对多连接 - 每个外键显示多个表的第一行

Question

我想使用以下查询从图库中获取前 5 行，但我只想从每行的 _gallery_gallery_items 中获取第一个匹配项。

例如，我想从画廊中获取狗画廊、猫画廊和后续画廊标题，但我只想从 _gallery_gallery_items 获取第一个匹配项，其中 _gallery_gallery_items.gallery_id = gallery.id。

所以结果应该是这样的：

id = 1
title = dog gallery
gallery_item_id = 70

id = 2
title = cat gallery
gallery_item_id = 102

id = 3
title = mouse gallery
gallery_item_id = 500

随后的 gallery_item_ids 不应出现在狗画廊中，例如。72, 72

到目前为止，这是我的查询：

SELECT gallery.id, gallery.title, _gallery_gallery_items.gallery_item_id
FROM gallery LEFT JOIN
_gallery_gallery_items ON gallery.id = _gallery_gallery_items.gallery_id

和表格：

画廊

id|title
1 |dog gallery
2 |cat gallery
3 |mouse gallery

_gallery_gallery_items

id|gallery_id|gallery_item_id
1|     1     |      70
2|     2     |      102
3|     1     |      71
4|     1     |      72
5|     3     |     500

Answer 1

MAX()我认为您可以使用和GROUP BY聚合功能来实现这一点

对于最后一个 gallery_item_id

SELECT a.id, a.title, MAX(b.gallery_item_id)
FROM gallery a
LEFT JOIN _gallery_gallery_items b 
ON a.id = b.gallery_id
GROUP BY b.gallery_id

对于第一个 gallery_item_id

SELECT a.id, a.title, MIN(b.gallery_item_id)
FROM gallery a
LEFT JOIN _gallery_gallery_items b 
ON a.id = b.gallery_id
GROUP BY b.gallery_id, a.title

Answer 2

试试这个：

SELECT
    g.id,
    g.title,
    (
        SELECT 
            gallery_item_id
        FROM
            _gallery_gallery_items gi
        WHERE 
            gi.gallery_id = g.id
        ORDER BY
            id
        LIMIT 1
    ) as gallery_item_id
FROM 
    gallery g

Answer 3

像这样它应该工作：

SELECT
  g.id,
  g.title,
  ggi.gallery_item_id
FROM gallery AS g
JOIN (
    SELECT 
      gallery_id, 
      MIN(gallery_item_id) 
    FROM _gallery_gallery_items 
    GROUP BY gallery_id
) AS ggi ON ggi.gallery_id = g.id

Answer 4

假设您有表 A 和一对多关系表 AB，而多表是 B：

Select *
From A table_a
Left Join (select * from AB group by A_id) table_ab on table_ab.A_id = table_a.id
#... (rest is yours)

谢谢。