Question

如何使用其他列的值，以便我可以使用所有相同类型的总和创建另一列。

e.g。

+---------+------+
|  name   | team |
+---------+------+
| Michael | red  |
| Lebron  | blue |
| Ben     | red  |
| Tiger   | red  |
| John    | blue |
+---------+------+

输出：

+---------+------+----------------+
|  name   | team | member_counter |
+---------+------+----------------+
| Michael | red  |              3 |
| Lebron  | blue |              2 |
| Ben     | red  |              3 |
| Tiger   | red  |              3 |
| John    | blue |              2 |
+---------+------+----------------+

你可以看到3个红色和2个蓝色。那么我还能使用之前列的值吗？ count（团队=上一栏）？作为member_counter？

Answer 1

您应该使用所有球队的子选择进行join

select name, team, teams.count 
 from names
 join (select team, count(1) as count from names group by team) as teams
using (team);

sqlfiddle

Answer 2

您可以JOIN到包含team的派生表计数：

SELECT m.name, m.team, t.cnt AS  member_counter
FROM mytable AS m
INNER JOIN (
   SELECT team, COUNT(*) AS cnt
   FROM mytable
   GROUP BY team ) t
ON m.team = t.team

这可能比相关的子查询表现得更好。

Demo here

Answer 3

如果你只想要这样的结果，你可以这样做：

SELECT T1.name,T1.team,T2.member_count
FROM TableName T1 JOIN
  (SELECT name,team,count(team) as member_count
   FROM TableName
   GROUP BY team) T2 ON T1.team=T2.team

结果：

name    team    member_count
----------------------------
Michael red     3
Lebron  blue    2
Ben     red     3
Tiger   red     3
John    blue    2

SQL Fiddle。

如果您要在表格中添加一列member_counter，可以执行以下操作：

ALTER TABLE TableName
ADD member_counter INT NULL


UPDATE TableName a 
   JOIN 
   (SELECT name,team,COUNT(team) as member_counter
    FROM TableName b
    GROUP BY team) b ON a.name = b.name  AND a.team=b.team
SET a.member_counter = b.member_counter

SQL Fiddle。

Answer 4

只做一个子查询：

        [HttpPost]
    public ActionResult Edit(ContactsEditViewModel viewModel)
    {
        if (ModelState.IsValid)
        {
            var Contacts = TheContactContext.tblContacts.Find(viewModel.ID);
            Contacts.Company = viewModel.Company;
            Contacts.Contact = viewModel.Contact;
            Contacts.Contact2 = viewModel.Contact2;
            Contacts.Email1 = viewModel.Email1;
            Contacts.Email2 = viewModel.Email2;
            Contacts.IsSupplier = viewModel.IsSupplier;
            Contacts.Telephone = viewModel.Telephone;
            Contacts.Link = viewModel.Website;
            Contacts.Notes = viewModel.Notes;

            TheContactContext.SaveChanges();
            return RedirectToAction("~/Contacts/Index");
        }
        return View(viewModel);
    }

修改

我检查了100万行。此查询的执行计划和已接受答案的查询具有99％的相同性。通过时间统计，两个查询的执行完全相同。使用IO统计信息，两个查询执行完全相同。而现在我无法理解为什么我被投票。唯一的区别是我的版本更短，更容易理解。

<强> EDIT2：

这是工作小提琴：

select *, (select count(*) from table t2 where t1.team = t2.team) as member_counter from table t1

MySQL：使用同一行上的列值进行计数

4 个答案: