如何在LINQ中按月将这些项目组合在一起？

Question

我有一个从数据库中提取数据的查询。它通过CheckDate对项目进行分组。我需要将CheckDate（标准日期时间）转换为月份，以便按月分组而不包含其他日期。我无法弄清楚如何做到这一点。

var query = from ech in this.dbContext.EmployeeCheckHistories
                    where ech.CompanyID == GlobalVariables.CompanyID &&
                    ech.Employees.ClientID == GlobalVariables.Client
                    group ech by ech.CheckDate into echG
                    orderby echG.Key descending
                    select echG;

Answer 1

  

我需要将CheckDate作为标准DateTime转换为月份

如果确实是DateTime，那么这应该可以解决问题。只需按CheckDate上的Month属性进行分组。

var query = from ech in this.dbContext.EmployeeCheckHistories
            where ech.CompanyID == GlobalVariables.CompanyID &&
                  ech.Employees.ClientID == GlobalVariables.Client
            group ech by ech.CheckDate.Month into echG
            orderby echG.Key descending
            select echG;

如果它实际上是Nullable<DateTime>，则需要检查该值是否存在，然后按月分组。

var query = from ech in this.dbContext.EmployeeCheckHistories
            where ech.CompanyID == GlobalVariables.CompanyID &&
                  ech.Employees.ClientID == GlobalVariables.Client &&
                  ech.CheckDate.HasValue
            group ech by ech.CheckDate.Value.Month into echG
            orderby echG.Key descending
            select echG;

Answer 2

您的查询 -

var query = from ech in this.dbContext.EmployeeCheckHistories
                    where ech.CompanyID == GlobalVariables.CompanyID &&
                    ech.Employees.ClientID == GlobalVariables.Client
                    group ech by ech.CheckDate into echG
                    orderby echG.Key descending
                    select echG;

可以简化为 -

var query = this.dbContext.EmployeeCheckHistories.Where(c=> 
     c.CompanyID == GlobalVariable.CompanyID 
     && c.Employees.ClientID == GlobalVariables.ClientID).GroupBy(d=> d.CheckDate.Month);