MySQL断开顺序列表中的分组依据
我有一个SQL:MySQL断开顺序列表中的分组依据,mysql,Mysql,我有一个SQL: SELECT c.name AS category_name, a.name, a.path, a.extension, a.width, a.height, a.server FROM categories c JOIN news_categories nc ON nc.categories_id = c.id JOIN news_attachments na ON nc.news_id = na.news_id JOI
SELECT
c.name AS category_name,
a.name,
a.path,
a.extension,
a.width,
a.height,
a.server
FROM categories c
JOIN news_categories nc ON nc.categories_id = c.id
JOIN news_attachments na ON nc.news_id = na.news_id
JOIN attachments a ON na.attachments_id = a.id
WHERE na.cover = 1
ORDER BY nc.news_id DESC
它告诉我一个正确的结果
+---------------+------------+---------------------+-----------+-------+--------+--------+
| category_name | name | path | extension | width | height | server |
+---------------+------------+---------------------+-----------+-------+--------+--------+
| Игры | c54950f283 | a6f00bcd/b25/cfaf2e | jpg | 450 | 416 | 1 |
| Игры | fa4249e534 | a6f00bcd/b89/cfaf2e | jpg | 400 | 250 | 1 |
| Игры | d7b009eeb7 | a6f00bcd/bfa/cfaf2e | jpg | 200 | 140 | 1 |
| Игры | 143cb7a38d | a6f00bcd/baa/cfaf2e | jpg | 200 | 140 | 1 |
| Игры | a70451eca7 | a6f00bcd/bc6/cfaf2e | jpg | 200 | 140 | 1 |
| Игры | 3ab8a0c051 | a6f00bcd/b5b/cfaf2e | jpg | 200 | 140 | 1 |
| Игры | 206255a07a | a6f00bcd/b98/cfaf2e | jpg | 200 | 140 | 1 |
| Игры | 86ef53ee1f | a6f00bcd/b93/cfaf2e | jpg | 200 | 140 | 1 |
| Игры | a50367f861 | a6f00bcd/b32/cfaf2e | jpg | 200 | 140 | 1 |
| Игры | dcfdd990c6 | a6f00bcd/b32/cfaf2e | jpg | 200 | 140 | 1 |
| Игры | 5a3df9f4ad | a6f00bcd/b34/cfaf2e | jpg | 200 | 140 | 1 |
| Игры | fb3ba0390d | a6f00bcd/b35/cfaf2e | jpg | 200 | 140 | 1 |
| Игры | 0e31ef118d | a6f00bcd/be5/cfaf2e | jpg | 457 | 249 | 1 |
| Игры | 82d41d9d62 | a6f00bcd/b97/cfaf2e | jpg | 457 | 249 | 1 |
+---------------+------------+---------------------+-----------+-------+--------+--------+
但如果我们按添加分组,那么它将不按顺序显示结果
SELECT
c.name AS category_name,
a.name,
a.path,
a.extension,
a.width,
a.height,
a.server
FROM categories c
JOIN news_categories nc ON nc.categories_id = c.id
JOIN news_attachments na ON nc.news_id = na.news_id
JOIN attachments a ON na.attachments_id = a.id
WHERE na.cover = 1
GROUP BY c.id ORDER BY nc.news_id DESC
那么结果就是
+---------------+------------+---------------------+-----------+-------+--------+--------+
| category_name | name | path | extension | width | height | server |
+---------------+------------+---------------------+-----------+-------+--------+--------+
| Игры | 82d41d9d62 | a6f00bcd/b97/cfaf2e | jpg | 457 | 249 | 1 |
+---------------+------------+---------------------+-----------+-------+--------+--------+
但一定是这样
+---------------+------------+---------------------+-----------+-------+--------+--------+
| category_name | name | path | extension | width | height | server |
+---------------+------------+---------------------+-----------+-------+--------+--------+
| Игры | c54950f283 | a6f00bcd/b25/cfaf2e | jpg | 450 | 416 | 1 |
+---------------+------------+---------------------+-----------+-------+--------+--------+
错误在哪里?按分组
将您的记录分组到c.id
。因此,每个c.id
值只能得到一行
在查询中,这个值有多行,MySQL将选择并显示一行:这里的最后一行
对于所需的输出,使用限制1
如果希望在结果集中有多个c.id
值(类别),则必须提供指向第一行的平均值,通常是主键
SELECT *
FROM Table;
+---------------+------------+----------+
| pk | category | data |
+---------------+------------+----------+
| 1 | red | a |
| 2 | red | b |
| 3 | red | c |
| 4 | red | d |
| 5 | red | e |
| 6 | red | f |
| 7 | green | g |
| 8 | green | h |
| 9 | green | i |
| 10 | green | j |
| 11 | blue | k |
| 12 | blue | l |
| 13 | blue | m |
| 14 | orange | n |
+---------------+------------+----------+
按分组
将您的记录分组到c.id
。因此,每个c.id
值只能得到一行
在查询中,这个值有多行,MySQL将选择并显示一行:这里的最后一行
对于所需的输出,使用限制1
如果希望在结果集中有多个c.id
值(类别),则必须提供指向第一行的平均值,通常是主键
SELECT *
FROM Table;
+---------------+------------+----------+
| pk | category | data |
+---------------+------------+----------+
| 1 | red | a |
| 2 | red | b |
| 3 | red | c |
| 4 | red | d |
| 5 | red | e |
| 6 | red | f |
| 7 | green | g |
| 8 | green | h |
| 9 | green | i |
| 10 | green | j |
| 11 | blue | k |
| 12 | blue | l |
| 13 | blue | m |
| 14 | orange | n |
+---------------+------------+----------+
您不正确地使用了分组方式
,其他数据库管理系统都会拒绝您的查询。有关详细信息,请阅读此和此。@user2058653GROUP BY
子句的意义是什么?我只需要每个类别一个结果,因为您错误地使用了GROUP BY
,其他DBMS都会拒绝您的查询。有关详细信息,请阅读此和此。@user2058653“分组依据”
子句的意义是什么?我每个类别只需要一个结果idI每个类别只需要一个结果id
,LIMIT
将只显示一个结果,但类别可以是(视频、游戏、软件)我需要为每个类别id提供一张图片,其中包含该类别中的最后一条新闻。请参考我的最后一句话。你能举个例子吗?我每个类别只需要一个结果id
,LIMIT
将只显示一个结果,但类别可以是(视频、游戏、软件)我需要一个单一的图像为每个类别id与最后的新闻在这个类别。请参考我的最后一句话。你能给一个例子请?
SELECT *
FROM Table t
WHERE pk = (SELECT MAX(pk)
FROM Table u
WHERE u.category = t.category);
+---------------+------------+----------+
| pk | category | data |
+---------------+------------+----------+
| 6 | red | f |
| 10 | green | j |
| 13 | blue | m |
| 14 | orange | n |
+---------------+------------+----------+