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);