Linq 多表连接查询join
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velue, velue = 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 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 c in a_join.DefaultIfEmpty()
select new {
d.ID,
UserID = (Int64?)d.UserID,
d.AvatarPic,
d.Name,
f.Title,
f.ContentDescribe,
Evaluate = ((int?)c.Evaluate ?? (int?)0)
}
Lambda表达式
.GroupJoin (
Functions,
d =>
new
{
FunctionID = d.FunctionID
},
f =>
new
{
FunctionID = f.ID
},
(d, b_join) =>
new
{
d = d,
b_join = b_join
}
)
.SelectMany (
temp0 => temp0.b_join.DefaultIfEmpty (),
(temp0, f) =>
new
{
temp0 = temp0,
f = f
}
)
.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))
}
),
temp1 =>
new
{
UserID = temp1.temp0.d.UserID
},
c =>
new
{
UserID = (Int64)(c.DoctorID)
},
(temp1, a_join) =>
new
{
temp1 = temp1,
a_join = a_join
}
)
.SelectMany (
temp2 => temp2.a_join.DefaultIfEmpty (),
(temp2, c) =>
new
{
ID = temp2.temp1.temp0.d.ID,
UserID = (Int64?)(temp2.temp1.temp0.d.UserID),
AvatarPic = temp2.temp1.temp0.d.AvatarPic,
Name = temp2.temp1.temp0.d.Name,
Title = temp2.temp1.f.Title,
ContentDescribe = temp2.temp1.f.ContentDescribe,
Evaluate = ((Int32?)(c.Evaluate) ?? (Int32?)0)
}
)