在Linq C#中按[ColumnName]选择COUNT (ColumnName)组

Question

我正在尝试使用C#对记录进行计数，并按Linq中的特定列对它们进行分组。下面是我想要转换为Linq的SQL语句。我希望将分组记录放到IEnumerable<object>或List<object>中，以便将列表传递给我的图表。

SELECT 
G.[Description] AS Grade,
COUNT(CASE WHEN A.ApplicationStatusId = 1 THEN 1 END) AS Pending,
COUNT(CASE WHEN A.ApplicationStatusId = 2 THEN 1 END) AS Accepted,
COUNT(CASE WHEN A.ApplicationStatusId = 3 THEN 1 END) AS Rejected,
COUNT(ApplicationId) AS Total
FROM [Application] A
LEFT JOIN Grade G ON A.GradeId = G.GradeId

GROUP BY G.[Description]

这些是上述SQL语句的SQL结果。

​

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我的课

public class Application : Audit
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int ApplicationId { get; set; }
        [DataType(DataType.DateTime)]
        public DateTime? ApplicationDate { get; set; }
        [StringLength(150)]
        public string PreviousSchoolName { get; set; }
        [StringLength(500)]
        public string PreviousSchoolReportUrl { get; set; }
        [StringLength(500)]
        public string PreviousSchoolTransferUrl { get; set; }
        public bool? Deleted { get; set; }

        #region Foreign Keys
        [ForeignKey("Leaner")]
        public int? LeanerId { get; set; }
        [ForeignKey("ApplicationStatus")]
        public int? ApplicationStatusId { get; set; }
        [ForeignKey("Grade")]
        public int? GradeId { get; set; }
        #endregion

        #region Navigation Properties
        public virtual Leaner Leaner { get; set; }
        public virtual ApplicationStatus ApplicationStatus { get; set; }
        public virtual Grade Grade { get; set; }
        #endregion

        public static Application GlobalApplication { get; set; }
        public static IEnumerable<Grade> Grades { get; set; }

        #region Dashboard Preperties - No Database Mapping
        [NotMapped]
        public int TotalApplications { get; set; }
        [NotMapped]
        public string GradeName { get; set; }
        #endregion
    }

public class Grade : LookupBase
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int GradeId { get; set; }


    }

public abstract class LookupBase
    {
        [StringLength(50)]
        public string Name { get; set; }
        [StringLength(250)]
        public string Description { get; set; }
    }

public abstract class Audit
    {
        [DataType(DataType.DateTime)]
        public DateTime? DateCreated { get; set; }
        [DataType(DataType.DateTime)]
        public DateTime? DateModified { get; set; }
        public int? CreatedByOnlineUserId { get; set; }
        public int? ModifiedByOnlineUserId { get; set; }
    }

Answer 1

当SQL聚合函数COUNT(expr)不能被null (与A.ApplicationStatusId列相同)、Count(expr != null)或Sum(expr != null ? 1 : 0)时映射到LINQ null(我更喜欢第二个，因为它似乎被EF翻译得更好)。

因此，等效的LINQ查询可以如下所示：

var query =
    from a in db.Application
    group a by a.Grade.Description into g
    select new
    {
        Grade = g.Key,
        Pending = g.Sum(a => a.ApplicationStatusId == 1 ? 1 : 0),
        Accepted = g.Sum(a => a.ApplicationStatusId == 2 ? 1 : 0),
        Rejected = g.Sum(a => a.ApplicationStatusId == 3 ? 1 : 0),
        Total = g.Count()
    };