MySQL选择COUNT（*）和具有最大日期的行

Question

我正在尝试选择按卖家分组的交易的COUNT（*）个及其最近的产品及其添加日期。 但是，由于某种原因，它似乎会继续按创建日期对交易进行排序，即使我尝试使用子查询来防止这种情况，该交易也会不断上升。 这是一个示例表：

------------------------------------------------
| ID | Provider |      URL        | CreateDate |
------------------------------------------------
| 1  | Prov1    | http://ex.com/1 | 2015-05-10 |
| 2  | Prov1    | http://ex.com/2 | 2015-06-10 |
| 3  | Prov1    | http://ex.com/3 | 2015-07-10 |
| 4  | Prov2    | http://ex.com/4 | 2015-05-10 |
| 5  | Prov2    | http://ex.com/5 | 2015-06-10 |
------------------------------------------------

我希望返回以下内容：

-----------------------------------------------------------
| ID | COUNT(*) | Provider |      URL        | CreateDate |
-----------------------------------------------------------
| 3  |    3     | Prov1    | http://ex.com/3 | 2015-07-10 |
| 5  |    2     | Prov2    | http://ex.com/5 | 2015-06-10 |
-----------------------------------------------------------

我当前的查询是：

SELECT ID,COUNT(*),Provider,CreateDate,URL
FROM (SELECT * FROM products ORDER BY CreateDate DESC)
GROUP BY Provider;

但这似乎不起作用。 有人有建议吗？

编辑谢谢大家的出色回答。 对我而言，极为奇怪的是，尽管它们似乎可以在SQL提琴中工作，但它们却在我的数据库服务器上失败。 例如，在服务器上使用以下命令可提供以下功能：

mysql> INSERT INTO products
-> (`ID`, `Provider`, `URL`, `CreateDate`)
-> VALUES
-> (1, 'Prov1', 'http://ex.com/1','2015-05-10'),
-> (2, 'Prov1', 'http://ex.com/2','2015-06-10'),
-> (3, 'Prov1', 'http://ex.com/3','2015-07-10'),
-> (4, 'Prov2', 'http://ex.com/4','2015-05-10'),
-> (5, 'Prov2', 'http://ex.com/5','2015-06-10')
-> ;
Query OK, 5 rows affected (0.06 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> SELECT ID,COUNT(*),Provider,CreateDate,URL
    -> FROM (SELECT * FROM products ORDER BY CreateDate DESC) t1
    -> GROUP BY Provider;
+------+----------+----------+---------------------+-----------------+
| ID   | COUNT(*) | Provider | CreateDate          | URL             |
+------+----------+----------+---------------------+-----------------+
|    1 |        3 | Prov1    | 2015-05-10 00:00:00 | http://ex.com/1 |
|    4 |        2 | Prov2    | 2015-05-10 00:00:00 | http://ex.com/4 |
+------+----------+----------+---------------------+-----------------+

在SQL小提琴上运行相同的东西时，按预期工作。 进一步，关于JOIN的评论应该起作用，但是我遇到同样的问题，它返回了意外的结果。 我的MySQL版本是5.5.37-MariaDB-34.0。 有什么想法吗？

Answer 1

SELECT p1.*, COUNT(*)
FROM products p1
JOIN
(
    select provider, max(CreateDate) as m_date
    from products 
    group by provider
) p2 on p1.provider = p2.provider and p1.CreateDate = p2.m_date

Answer 2

您需要在想要mysql返回计数内容的地方指定字段名称：

select count(*) as nro from table;

在您的情况下：

SELECT ID,COUNT(*) as nro,Provider,CreateDate,URL
FROM (SELECT * FROM products ORDER BY CreateDate DESC)
GROUP BY Provider;

但对于所有解决方案，您都需要为此查询“ SELECT * FROM products ORDER BY CreateDate DESC”的这一部分创建别名

CREATE VIEW p AS (SELECT * FROM products ORDER BY CreateDate DESC);

第二个查询：

  SELECT ID,COUNT(*) as nro,Provider,CreateDate,URL
  FROM (p)
  GROUP BY Provider;

必须工作。

Answer 3

我是在SQL Server中完成的，但mysql中的查询有效。

Create TABLE #Products(
Id int,
Provider varchar(50),
URL Varchar(300),
Createdate varchar(50)
)


INSERT INTO #Products

SELECT 1,'Prov1','http://ex.com/1 ','2015-05-10'  UNION ALL
SELECT 2,'Prov1','http://ex.com/2 ','2015-06-10'  UNION ALL
SELECT 3,'Prov1','http://ex.com/3 ','2015-07-10'  UNION ALL
SELECT 4,'Prov2','http://ex.com/4 ','2015-05-10'  UNION ALL
SELECT 5,'Prov2','http://ex.com/5 ','2015-06-10' 




SELECT p1.id, p2.Provider,p2.m_date,p1.url, COUNT(*)
FROM #products p1
JOIN
(
    select  provider, max(CreateDate) as m_date
    from #products 
    group by provider
) p2 on p1.provider = p2.provider and p1.CreateDate = p2.m_date
GROUP BY p1.id, p2.Provider,p2.m_date,p1.url