我有这个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
JOIN attachments a ON na.attachments_id = a.id
WHERE na.cover = 1
ORDER BY nc.news_id DESC
它显示了正确的结果
+---------------+------------+---------------------+-----------+-------+--------+--------+
| 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 |
+---------------+------------+---------------------+-----------+-------+--------+--------+
但如果我们添加GROUP BY
,则会显示结果
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
然后结果将是
+---------------+------------+---------------------+-----------+-------+--------+--------+
| category_name | name | path | extension | width | height | server |
+---------------+------------+---------------------+-----------+-------+--------+--------+
| Игры | 82d41d9d62 | a6f00bcd/b97/cfaf2e | jpg | 457 | 249 | 1 |
+---------------+------------+---------------------+-----------+-------+--------+--------+
但它一定是这样的
+---------------+------------+---------------------+-----------+-------+--------+--------+
| category_name | name | path | extension | width | height | server |
+---------------+------------+---------------------+-----------+-------+--------+--------+
| Игры | c54950f283 | a6f00bcd/b25/cfaf2e | jpg | 450 | 416 | 1 |
+---------------+------------+---------------------+-----------+-------+--------+--------+
哪里出错了?
答案 0 :(得分:1)
GROUP BY
将您的记录分组到c.id
。因此,每个c.id
值只能获得一行。
碰巧,你的查询中有多行代表这个值,MySQL会选择并显示一行:最后一行。
对于所需的输出使用LIMIT 1
。
如果您想在结果集中使用多个c.id
值(类别),则必须提供定位first
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 |
+---------------+------------+----------+
SELECT *
FROM Table
GROUP BY category;
+---------------+------------+----------+
| pk | category | data |
+---------------+------------+----------+
| 1 | red | a |
| 7 | green | g |
| 11 | blue | k |
| 14 | orange | n |
+---------------+------------+----------+
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 |
+---------------+------------+----------+