使用 Linq 进行多表联接查询的方法

在查询语言中，通常需要使用联接操作。在 LINQ 中，可以通过 join 子句实现联接操作。join 子句可以将来自不同源序列，并且在对象模型中没有直接关系(数据库表之间没有关系)的元素相关联，唯一的要求是每个源中的元素需要共享某个可以进行比较，以判断是否相等的值。

在 LINQ 中，join 子句可以实现 3 种类型的联接分别是内部联接、分组联接和左外部联接。

1、内部连接(相对于sql：join | inner join)

格式：join element in dataSource on exp1 equals exp2

int[] intAry1 = {5, 15, 25, 30, 33, 40};//创建整数数组 intAry1 作为数据源
int[] intAry2 = {10, 20, 30, 50, 60, 70, 80};//创建整数数组 intAry2 作为数据源
//查询 query1 使用 join 子句从两个数据源获取数据
//演示内部联接的使用
var query1 =
from val1 in intAry1
join val2 in intAry2 on val1%5 equals val2%15
select new {VAL1=val1, VAL2=val2};

2、分组连接

格式： join element in dataSource on exp1 equals exp2 into grpName

其中，into 关键字表示将这些数据分组并保存到 grpName 中，grpName 是保存一组数据的集合。（感觉和sql不同，sql查询的结果是平面矩形的，而linq则是平面树形的，意思是像对象的元素也是个对象）

int[] intAry1 = { 5, 15, 25, 30, 33, 40 };//创建整数数组 intAry1 作为数据源
int[] intAry2 = { 10, 20, 30, 50, 60, 70, 80 };//创建整数数组 intAry2 作为数据源
//查询 query1 使用 join 子句从两个数据源获取数据
//演示分组联接的使用
var query1 =
from val1 in intAry1
join val2 in intAry2 on val1 % 5 equals val2 % 15 into val2Grp
select new { VAL1 = val1, VAL2GRP = val2Grp};

3、左外部联接 (相对于sql：left join | left outer join)

第三种联接是左外部联接，它返回第一个集合中的所有元素，无论它是否在第二个集合中有相关元素。在 LINQ 中，通过对分组联接的结果调用 DefaultIfEmpty()方法来执行左外部联接。DefaultIfEmpty()方法从列表中获取指定元素。如果列表为空，则返回默认值。

int[] intAry1 = { 5, 15, 23, 30, 33, 40 };//创建整数数组 intAry1 作为数据源
int[] intAry2 = { 10, 20, 30, 50, 60, 70, 80 };//创建整数数组 intAry2 作为数据源
//查询 query1 使用 join 子句从两个数据源获取数据
//演示左联接的使用
var query1 =
from val1 in intAry1
join val2 in intAry2 on val1 % 5 equals val2 % 15 into val2Grp
from grp in val2Grp.DefaultIfEmpty()
select new { VAL1 = val1, VAL2GRP = grp };

查询方法Lambda示例（GroupJoin）：

原形：https://msdn.microsoft.com/zh-cn/library/bb534297(v=vs.105).aspx

public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
    this IEnumerable<TOuter> outer,
    IEnumerable<TInner> inner,
    Func<TOuter, TKey> outerKeySelector,
    Func<TInner, TKey> innerKeySelector,
    Func<TOuter, IEnumerable<TInner>, TResult> resultSelector
)

重载

public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
    this IEnumerable<TOuter> outer,
    IEnumerable<TInner> inner,
    Func<TOuter, TKey> outerKeySelector,
    Func<TInner, TKey> innerKeySelector,
    Func<TOuter, IEnumerable<TInner>, TResult> resultSelector,
    IEqualityComparer<TKey> comparer
)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LinqDemo2
{
    /// <summary>
    /// 学生实体
    /// </summary>
    public class Student
    {
        public int StudentId { get; set; }
        public string StudentName { get; set; }
        public int StandardId { get; set; }//水平
    }

    /// <summary>
    /// 水平/等级
    /// </summary>
    public class Standard
    {
        public int StandardId { get; set; }
        public string StandardName { get; set; }//
    }
    class Program
    {
        static void Main(string[] args)
        {
            #region 数据源
            IList<Student> studentList = new List<Student>()
            {
                new Student() {StudentId = 1, StudentName = "John", StandardId = 1},
                new Student() {StudentId = 2, StudentName = "Moin", StandardId = 1},
                new Student() {StudentId = 3, StudentName = "Bill", StandardId = 2},
                new Student() {StudentId = 4, StudentName = "Ram", StandardId = 2},
                new Student() {StudentId = 5, StudentName = "Ron"}
            };
            IList<Standard> standardList = new List<Standard>()
            {
                new Standard() {StandardId = 1, StandardName = "优秀"},
                new Standard() {StandardId = 2, StandardName = "中等"},
                new Standard() {StandardId = 3, StandardName = "差生"}
            };
            #endregion

            //查询公式
            var groupJoin = standardList.GroupJoin(studentList,
                standard => standard.StandardId,
                student => student.StandardId,
                (standard, studentGroup) => new
                {
                    StandarFullName = standard.StandardName,
                    Students = studentGroup
                });//感觉和字典类型一样，一个key，对应一个value, value = IEnumerable<Student>

            //执行查询
            foreach (var item in groupJoin)
            {
                Console.WriteLine(item.StandarFullName);

                foreach (var student in item.Students)
                {
                    Console.WriteLine(student.StudentName);
                }
            }

            /* 输出：
             * 
            优秀
            John
            Moin
            中等
            Bill
            Ram
            差生
            */


        }
    }
}

示例：分页查询

            var page = 1;
            var pageSize = 10;
            var query = (from user in db.Set<User>()
                         join userRole in db.Set<UserRole>() on user.Id equals userRole.UserId
                         join rolePrivilege in db.Set<RolePrivilege>() on userRole.RoleId equals rolePrivilege.RoleId
                         join priviege in db.Set<Privilege>() on rolePrivilege.PrivilegeId equals priviege.Id
                         join role in db.Set<Role>() on userRole.RoleId equals role.Id
                         where user.Id == 1 && userRole.RoleId == 1
                         orderby user.Id descending
                         select new
                         {
                             user.Id,
                             userRole.RoleId,
                             user.Username,
                             PrivilegeName = priviege.Name,
                             RoleName = role.Name
                         }).Skip((page - 1) * pageSize).Take(pageSize);

LINQ，EF联合查询join

public object GetListAdmin()  
        {  
            //return db_C56.Admins  
            //   .Where(a => a.Status != "D").ToList();  
  
            var query1 = db_C56.Admins.Join(db_C56.Area, a => a.AreaID, ar => ar.ID, (a, ar) => new  
            {  
                userName = a.UserName,  
                pwd = a.Password,  
                dName = a.DisplayName,  
                areaId = a.AreaID,  
                hasNode = a.HasNode,  
                roleName = a.RoleName,  
                status = a.Status,  
                areaName = ar.Name  
            });  
  
            var query = from a in db_C56.Admins  
                        join ar in db_C56.Area  
                        on a.AreaID equals ar.ID  
                        where a.Status != "D"  
                        select new  
                        {  
                            userName = a.UserName,  
                            pwd = a.Password,  
                            dName = a.DisplayName,  
                            areaId = a.AreaID,  
                            hasNode = a.HasNode,  
                            roleName = a.RoleName,  
                            status = a.Status,  
                            areaName = ar.Name  
                        };  
            return query.ToList().Select(C => new Admin  
            {  
                UserName = C.userName,  
                Password = C.pwd,  
                DisplayName = C.dName,  
                AreaID = C.areaId,  
                AreaPath = C.areaName,  
                HasNode = C.hasNode,  
                RoleName = C.roleName,  
                Status = C.status,  
            });  
        }  

from v in Pdt_Versions  
join t in Tb_TypeDics   
on v.TypeName equals t.TypeName into ignored  
from i in ignored.DefaultIfEmpty()  
where v.Status != "D"  
select new   
{  
ID = v.ID,  
VersionName = v.VersionName,  
VersionCode = v.VersionCode,  
DownloadName = v.DownloadName,  
DownloadURL = v.DownloadURL,  
VType = v.VType,  
TypeName = v.TypeName,  
DisplyTypeName = i.DisplyTypeName,  
}  

Linq 多层嵌套查询

var query1 = from p in dbContent.PostService  
             where p.post_type == "product" &&  
                 (from ot1 in dbContent.OrderItemmetaService  
                  where  
                      (from ot2 in dbContent.OrderItemsService  
                       where ot2.order_item_type == "line_item" &&  
                           (from p1 in dbContent.PostService  
                            where p1.post_type == "shop_order" && p1.post_author == userid && p1.post_status == "wc-completed"  
                            select p1.ID).Contains(ot2.order_id)  
                       select ot2.order_item_id).Contains(ot1.meta_id)  
                  select ot1.meta_value).Contains(p.ID)  
             select new  
             {  
                 id = p.ID,  
                 name = p.post_title  
             };  


var query2 = dbContent.PostService.Where(p =>  
    p.post_type == "product" &&  
    (dbContent.OrderItemmetaService.Where(ot1 =>  
        (dbContent.OrderItemsService.Where(ot2 =>  
            ot2.order_item_type == "line_item" && (dbContent.PostService.Where(p1 =>  
                p1.post_type == "shop_order" && p1.post_author == userid && p1.post_status == "wc-completed").Select(p1 => p1.ID).Contains(ot2.order_item_id))  
                ).Select(ot2 => ot2.order_item_id).Contains(ot1.meta_id))  
                ).Select(ot1 => ot1.meta_value).Contains(p.ID))  
                ).Select(p => new  
                {  
                    id = p.ID,  
                    name = p.post_title  
                }).ToList();  

Left Join 查询

from d in Doctors
join c in (
(from t in Commentaries where t.State != 'D' group t by new { t.DoctorID } into g 
select new {
DoctorID = (Int64?)g.Key.DoctorID,
Total = (Int32?)g.Sum(p => p.Rating),
Evaluate = (System.Double?)g.Average(p => p.Rating)
})) on new { UserID = d.UserID } equals new { UserID = (Int64)c.DoctorID }into a_join


from p in a_join.DefaultIfEmpty()
select new {
  d.ID,
  UserID = (Int64?)d.UserID,
  d.Name,
  Evaluate = ((int?)p.Evaluate ?? (int?)0)
}

Lambda表达式

Doctors
   .GroupJoin (
      Commentaries
         .Where (t => ((Int32)(t.State) != 68))
         .GroupBy (
            t => 
               new  
               {
                  DoctorID = t.DoctorID
               }
         )
         .Select (
            g => 
               new  
               {
                  DoctorID = (Int64?)(g.Key.DoctorID), 
                  Total = (Int32?)(g.Sum (p => p.Rating)), 
                  Evaluate = (Double?)(g.Average (p => p.Rating))
               }
         ), 
      d => 
         new  
         {
            UserID = d.UserID
         }, 
      c => 
         new  
         {
            UserID = (Int64)(c.DoctorID)
         }, 
      (d, a_join) => 
         new  
         {
            d = d, 
            a_join = a_join
         }
   )
   .SelectMany (
      temp0 => temp0.a_join.DefaultIfEmpty (), 
      (temp0, p) => 
         new  
         {
            ID = temp0.d.ID, 
            UserID = (Int64?)(temp0.d.UserID), 
            Name = temp0.d.Name, 
            Evaluate = ((Int32?)(p.Evaluate) ?? (Int32?)0)
         }
   )

======================================================================

多个left join

from d in Doctors
join f in Functions on new { FunctionID = d.FunctionID } equals new { FunctionID = f.ID } into b_join



from f in b_join.DefaultIfEmpty()
join c in (
(from t in Commentaries whe