MySQL-在选择时获取行号

如果项目已排序，是否可以运行select语句并获取行号？

我有一个这样的表：

mysql> describe orders;
+-------------+---------------------+------+-----+---------+----------------+
| Field       | Type                | Null | Key | Default | Extra          |
+-------------+---------------------+------+-----+---------+----------------+
| orderID     | bigint(20) unsigned | NO   | PRI | NULL    | auto_increment |
| itemID      | bigint(20) unsigned | NO   |     | NULL    |                |
+-------------+---------------------+------+-----+---------+----------------+

然后，我可以运行此查询以按ID获取订单数量：

SELECT itemID, COUNT(*) as ordercount
FROM orders
GROUP BY itemID ORDER BY ordercount DESC;

这样，我就可以itemID对表格中的每个表格进行计数：

+--------+------------+
| itemID | ordercount |
+--------+------------+
|    388 |          3 |
|    234 |          2 |
|   3432 |          1 |
|    693 |          1 |
|   3459 |          1 |
+--------+------------+

我也想获得行号，所以我可以知道那itemID=388是第一行，234第二行，等等（本质上是订单的排名，而不仅仅是原始计数）。我知道我可以在返回结果集时用Java做到这一点，但是我想知道是否有一种方法可以纯粹在SQL中处理它。

更新资料

设置等级会将其添加到结果集中，但顺序不正确：

mysql> SET @rank=0;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @rank:=@rank+1 AS rank, itemID, COUNT(*) as ordercount
    -> FROM orders
    -> GROUP BY itemID ORDER BY rank DESC;
+------+--------+------------+
| rank | itemID | ordercount |
+------+--------+------------+
|    5 |   3459 |          1 |
|    4 |    234 |          2 |
|    3 |    693 |          1 |
|    2 |   3432 |          1 |
|    1 |    388 |          3 |
+------+--------+------------+
5 rows in set (0.00 sec)

看看这个。

将查询更改为：

SET @rank=0;
SELECT @rank:=@rank+1 AS rank, itemID, COUNT(*) as ordercount
  FROM orders
  GROUP BY itemID
  ORDER BY ordercount DESC;
SELECT @rank;

最后选择是您的数量。

SELECT @rn:=@rn+1 AS rank, itemID, ordercount
FROM (
  SELECT itemID, COUNT(*) AS ordercount
  FROM orders
  GROUP BY itemID
  ORDER BY ordercount DESC
) t1, (SELECT @rn:=0) t2;

Swamibebop的解决方案有效，但是通过利用table.*语法，我们可以避免重复内部的列名，select并获得更简单/更简短的结果：

SELECT @r := @r+1 , 
       z.* 
FROM(/* your original select statement goes in here */)z, 
(SELECT @r:=0)y;

这样就可以给您：

SELECT @r := @r+1 , 
       z.* 
FROM(
     SELECT itemID, 
     count(*) AS ordercount
     FROM orders
     GROUP BY itemID
     ORDER BY ordercount DESC
    )z,
    (SELECT @r:=0)y;

您可以使用MySQL变量来做到这一点。这样的事情应该起作用（尽管它包含两个查询）。

SELECT 0 INTO @x;

SELECT itemID, 
       COUNT(*) AS ordercount, 
       (@x:=@x+1) AS rownumber 
FROM orders 
GROUP BY itemID 
ORDER BY ordercount DESC; 

现在它已内置在MySQL 8.0和MariaDB 10.2中：

SELECT
  itemID, COUNT(*) as ordercount,
  ROW_NUMBER OVER (PARTITION BY itemID ORDER BY rank DESC) as rank
FROM orders
GROUP BY itemID ORDER BY rank DESC