在单个查询中，我想在两个不同表格的两个不同列上public class Program
{
public static void Main()
{
new Program().Execute();
}
public void Execute()
{
// lock objects
this.fixErrorLock = new object();
this.isLoggingInLock = new object();
var objectsToIterate = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 };
objectsToIterate.AsParallel().ForAll(this.DoWork);
}
private object isLoggingInLock;
private object fixErrorLock;
private bool isLoggingIn;
public bool IsThereAnyThreadLoggingIn()
{
lock (this.isLoggingInLock)
{
// If no thread is logging-in, the one who asked is going to log-in
if (!this.isLoggingIn)
this.isLoggingIn = true;
return this.isLoggingIn;
}
}
public void DoWork(int myParam)
{
try
{
if (myParam % 4 == 0)
throw new Exception();
}
catch (Exception ex)
{
// Is the equivalent of 'is the first thread to hit here?'
bool canLogIn = this.IsThereAnyThreadLoggingIn();
// Every thread with error will stop here
lock (fixErrorLock)
{
// But only the first one will do the login process again
if (canLogIn)
{
// Inside the login method the variable responsible for the 'isFirstThread' is restored to false
this.LogIn();
}
}
this.DoWork(myParam-1);
}
}
public void LogIn()
{
Thread.Sleep(100);
lock (this.isLoggingInLock)
{
// The login is done
this.isLoggingIn = false;
}
}
}
和COUNT。
问题是GROUP_CONCAT返回的数字乘以COUNT中不同项目的数量，而GROUP_CONCAT结果中，每个不同的项目乘以数字COUNT应该返回。
以下是查询：
GROUP_CONCAT这是一个SQL Fiddle所以这个更清晰/可测试。
结果看起来像这样：
SELECT e.id, GROUP_CONCAT(c.id SEPARATOR ',') AS category_ids, COUNT(a.id) AS numberAttenders, e.event_capacity
FROM events e
LEFT JOIN attendees a ON a.event_id=e.id,
categories c,
event_categories ec
WHERE e.id=ec.event_id
AND ec.category_id=c.id
GROUP BY e.id
HAVING numberAttenders < e.event_capacity OR e.event_capacity=0
当我想要这样的东西时：
id 1
category_ids 1,2,1,2
numberAttenders 4
event_capacity 10
我和id 1
category_ids 1,2
numberAttenders 2
event_capacity 10
玩了一点但是没有成功的结果。
答案 0 :(得分：2)
您将获得每个活动的与会者和类别的笛卡尔积。最好的解决方案是在进行连接之前聚合表：
SELECT e.id, ec.category_ids, a.NumAttendees, e.event_capacity
FROM events e LEFT JOIN
(SELECT a.event_id, COUNT(*) as NumAttendees
FROM attendees a
GROUP BY a.event_id
) a
ON a.event_id = e.id LEFT JOIN
(SELECT ec.event_id, GROUP_CONCAT(c.id SEPARATOR ',') as category_ids
FROM event_categories ec JOIN
categories c
ON ec.category_id = c.id
GROUP BY ec.event_id
) ec
ON e.id=ec.event_id
HAVING NumAttendees < e.event_capacity OR e.event_capacity = 0;
Here是一个SQL小提琴。