linq用法整理
普通查询
var highScores = from student in students where student.ExamScores[exam] > score select new {Name = student.FirstName, Score = student.ExamScores[exam]}; Group by
var queryLastNames = from student in students group student by student.LastName into newGroup orderby newGroup.Key select newGroup;
有条件Group by
var queryGroupByAverages = from student in students group new { student.FirstName, student.LastName } by student.ExamScores.Average() > 75 into studentGroup select studentGroup; var queryHighScoreGroups = from student in students group student by new { FirstLetter = student.LastName[0], Score = student.ExamScores[0] > 85 } into studentGroup orderby studentGroup.Key.FirstLetter select studentGroup; 嵌套Group
var queryNestedGroups = from student in students group student by student.Year into newGroup1 from newGroup2 in (from student in newGroup1 group student by student.LastName) group newGroup2 by newGroup1.Key;
Inner Joins
- Simple key join 简单键值join
var query = from person in people join pet in pets on person equals pet.Owner select new { OwnerName = person.FirstName, PetName = pet.Name }; - Composite key join 多条件join
IEnumerablequery = from employee in employees join student in students on new { employee.FirstName, employee.LastName } equals new { student.FirstName, student.LastName } select employee.FirstName + " " + employee.LastName;
- Multiple join 多条件join
var query = from person in people join cat in cats on person equals cat.Owner join dog in dogs on new { Owner = person, Letter = cat.Name.Substring(0, 1) } equals new { dog.Owner, Letter = dog.Name.Substring(0, 1) } select new { CatName = cat.Name, DogName = dog.Name }; - Inner join by using grouped join 使用分组连接的内联
var query1 = from person in people join pet in pets on person equals pet.Owner into gj from subpet in gj select new { OwnerName = person.FirstName, PetName = subpet.Name }; var query2 = from person in people join pet in pets on person equals pet.Owner select new { OwnerName = person.FirstName, PetName = pet.Name }; left outer joins 左外部联接
var query = from person in people join pet in pets on person equals pet.Owner into gj from subpet in gj.DefaultIfEmpty() select new { person.FirstName, PetName = subpet?.Name ?? String.Empty }; join 子句的结果进行排序
var groupJoinQuery2 = from category in categories join prod in products on category.ID equals prod.CategoryID into prodGroup orderby category.Name select new { Category = category.Name, Products = from prod2 in prodGroup orderby prod2.Name select prod2 }; Join by using composite keys 多条件 join
var query = from o in db.Orders from p in db.Products join d in db.OrderDetails on new {o.OrderID, p.ProductID} equals new {d.OrderID, d.ProductID} into details from d in details select new { o.OrderID, p.ProductID, d.UnitPrice }; 在查询表达式中处理 null 值Handle null values in query expressions
var query1 = from c in categories where c != null join p in products on c.ID equals p?.CategoryID select new { Category = c.Name, Name = p.Name }; void TestMethod(Northwind db){ var query = from o in db.Orders join e in db.Employees on o.EmployeeID equals (int?)e.EmployeeID select new { o.OrderID, e.FirstName };}