c# - 如何从 MySQL 中选择 sum(max remain(x+y+z)) 来捕获 sum(1200+1300+1400)

Question

如何从 MySQL中选择sum(max remain(x + y + z))，来捕获sum(1200+1300+1400)？

id | user | remain
-----------------
1  | x   | 1000
----------------
2  | x   | 1200
----------------
3  | y   | 1100
----------------
4  | y   | 1300
----------------
5  | z   | 1200
----------------
5  | z   | 1400
----------------
using (SqlConnection cn = new SqlConnection(Class1.x))
        {
            cn.Open();
            string cm1 = "select sum(max remain(all users)) as 'total' from item_new_company";
            using (SqlCommand cmd = new SqlCommand(cm1, cn))
            {
                using (SqlDataReader dr = cmd.ExecuteReader())
                { dr.Read(); tot5 = dr["total"].ToString(); }
            }
        }

Answer 1

像这样:

SELECT SUM(MaxRemain) TotalOfMaxRemains
FROM
(
    SELECT MAX(remain) AS MaxRemain
    FROM item_new_company
    GROUP BY user
) AS t;

SQL Fiddle Demo

结果:

| TOTALOFMAXREMAINS |
---------------------
|              3900 |

---

子查询:

SELECT MAX(remain) AS MaxRemain
FROM item_new_company
GROUP BY user

使用 GROUP BY user 和 MAX(remain)，将为每个 user 提供 remain 的最大值>，然后在外部查询中，SUM 将为您提供总数。

---

更新

对于 SQL Server，前面的查询应该可以正常工作，但还有另一种方法:

WITH CTE AS
(
  SELECT *, ROW_NUMBER() OVER(PARTITION BY [user] 
                              ORDER BY id DESC) AS rownum 
  FROM item_new_company 
) 
SELECT SUM(remain) AS Total 
FROM CTE 
WHERE rownum = 1;

SQL Fiddle Demo