SELECT COUNT (ColumnName) Group By [ColumnName] in Linq C#

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。 这是我想转换为 Linq 的 SQL 语句。我想将分组记录放入IEnumerable<object> 或List<object>，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。

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expr 不能是null（您的A.ApplicationStatusId 列就是这种情况）、Count(expr != null) 或Sum(expr != null ? 1 : 0) 时，SQL 聚合函数 COUNT(expr) 映射到 LINQ Count()，否则（我更喜欢第二个，因为它似乎被 EF 翻译得更好）。

所以等效的 LINQ 查询可能是这样的：

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()
    };