SQL查询：仅左连接返回交叉点

我一直在努力解决我认为是一个简单的SQL查询：

SELECT 
  resource.firstname, 
  resource.lastname, 
  resource.fte, 
  project.name as project_name, 
  role.name as role_name, 
  assignment.startdate, 
  assignment.enddate, 
  assignment.numberofdaysperweek
FROM 
  resource
LEFT JOIN assignment ON resource.id = assignment.resource_id AND assignment.enddate < now()
LEFT JOIN project ON project.id = assignment.project_id 
LEFT JOIN role ON role.id = assignment.role_id

因此，我在这个查询中有4个表：资源被分配给项目，并被赋予该项目的特定角色

---

此查询的问题是，它将返回任何资源+任何分配已过期的资源的数据。enddate要将分配限制为每个资源的最新分配，你需要找到每个资源id的最新开始日期，并将内部连接返回分配表

SELECT  assignment.*
FROM    assignment
        INNER JOIN
        (   SELECT  assignment.resource_id, 
                    MAX(assignment.StartDate) AS StartDate
            FROM    assignment
            GROUP BY assignment.resource_id
        ) MaxAssignment
            ON assignment.resource_id = MaxAssignment.resource_id
            AND assignment.StartDate = MaxAssignment.StartDate;

由于您只想左键连接到该查询，因此需要将上面的全部内容移动到子查询中，并将整个子查询左键连接回主查询：

SELECT  resource.firstname, 
        resource.lastname, 
        resource.fte, 
        project.name as project_name, 
        role.name as role_name, 
        assignment.startdate, 
        assignment.enddate, 
        assignment.numberofdaysperweek
FROM    resource
        LEFT JOIN 
        (   SELECT  assignment.*
            FROM    assignment
                    INNER JOIN
                    (   SELECT  assignment.resource_id, 
                                MAX(assignment.StartDate) AS StartDate
                        FROM    assignment
                        GROUP BY assignment.resource_id
                    ) MaxAssignment
                        ON assignment.resource_id = MaxAssignment.resource_id
                        AND assignment.StartDate = MaxAssignment.StartDate
        ) assignment
            ON resource.id = assignment.resource_id 
            AND assignment.enddate < now()
        LEFT JOIN project 
            ON project.id = assignment.project_id 
        LEFT JOIN role 
            ON role.id = assignment.role_id;

---

要将分配限制为每个资源的最新分配，您需要找到每个资源的最新开始日期，并将内部连接返回分配表

SELECT  assignment.*
FROM    assignment
        INNER JOIN
        (   SELECT  assignment.resource_id, 
                    MAX(assignment.StartDate) AS StartDate
            FROM    assignment
            GROUP BY assignment.resource_id
        ) MaxAssignment
            ON assignment.resource_id = MaxAssignment.resource_id
            AND assignment.StartDate = MaxAssignment.StartDate;

由于您只想左键连接到该查询，因此需要将上面的全部内容移动到子查询中，并将整个子查询左键连接回主查询：

SELECT  resource.firstname, 
        resource.lastname, 
        resource.fte, 
        project.name as project_name, 
        role.name as role_name, 
        assignment.startdate, 
        assignment.enddate, 
        assignment.numberofdaysperweek
FROM    resource
        LEFT JOIN 
        (   SELECT  assignment.*
            FROM    assignment
                    INNER JOIN
                    (   SELECT  assignment.resource_id, 
                                MAX(assignment.StartDate) AS StartDate
                        FROM    assignment
                        GROUP BY assignment.resource_id
                    ) MaxAssignment
                        ON assignment.resource_id = MaxAssignment.resource_id
                        AND assignment.StartDate = MaxAssignment.StartDate
        ) assignment
            ON resource.id = assignment.resource_id 
            AND assignment.enddate < now()
        LEFT JOIN project 
            ON project.id = assignment.project_id 
        LEFT JOIN role 
            ON role.id = assignment.role_id;

---

大多数数据库支持行数功能。使用此函数，您可以通过以下方式处理此问题：

SELECT 
  resource.firstname, 
  resource.lastname, 
  resource.fte, 
  project.name as project_name, 
  role.name as role_name, 
  assignment.startdate, 
  assignment.enddate, 
  assignment.numberofdaysperweek
FROM 
  resource
LEFT JOIN (select a.*,
                  row_number() over (partition by resource_id order by enddate) as seqnum
           from assignment a
           where assignment.enddate < now()
          ) assignment
     ON resource.id = assignment.resource_id AND seqnum = 1
LEFT JOIN project ON project.id = assignment.project_id 
LEFT JOIN role ON role.id = assignment.role_id;

---

row_number函数为每个值的resource_id分区内的分配按resource_id分配一个序列号。然后对这些分配进行排序，以便具有最新enddate的分配按enddate获得1 order值。

大多数数据库都支持row_number函数。使用此函数，您可以通过以下方式处理此问题：

SELECT 
  resource.firstname, 
  resource.lastname, 
  resource.fte, 
  project.name as project_name, 
  role.name as role_name, 
  assignment.startdate, 
  assignment.enddate, 
  assignment.numberofdaysperweek
FROM 
  resource
LEFT JOIN (select a.*,
                  row_number() over (partition by resource_id order by enddate) as seqnum
           from assignment a
           where assignment.enddate < now()
          ) assignment
     ON resource.id = assignment.resource_id AND seqnum = 1
LEFT JOIN project ON project.id = assignment.project_id 
LEFT JOIN role ON role.id = assignment.role_id;

---

row_number（行编号）函数为每个值的资源id分区内的分配按资源id分配一个序列号。然后对这些分配进行排序，以便具有最新结束日期的分配按结束日期获得1个值。

如何定义最后一个分配？是否也基于结束日期？开始日期？没有提到其他专栏？同一个资源不可能有两个工作分配重叠，所以，最后一个enddate，最后一个startdate，是相同的。资源的最新分配条目也是相同的。如何定义上次分配？是否也基于结束日期？开始日期？没有提到其他专栏？同一个资源不可能有两个工作分配重叠，所以，最后一个enddate，最后一个startdate，是相同的。资源的最新分配条目也是相同的。使用select，我从pour la table«分配»第23行：resource.id=assignment.resource_id上的分配中得到：ERREUR:entrée manquante de la子句和as。。。转换为：错误：表赋值的FROM子句中缺少条目我拼错了子查询上的别名赋值。我已经改正了。我的错，我以为我已经全部检查过了。谢谢，它工作得很好：使用select，我从pour la table«assignment»第23行：assignment ON resource.id=assignment.resource_id和as中获得：ERREUR:entrée manquante de la子句。。。转换为：错误：表赋值的FROM子句中缺少条目我拼错了子查询上的别名赋值。我已经改正了。我的错，我以为我已经全部检查过了。谢谢，很好用：