LINQ左连接和右连接
我需要帮助，
我有两个数据表称为A和B，我需要从A的所有行和B的匹配行
例如：
A: B: User | age| Data ID | age|Growth 1 |2 |43.5 1 |2 |46.5 2 |3 |44.5 1 |5 |49.5 3 |4 |45.6 1 |6 |48.5 我需要输出：
User | age| Data |Growth ------------------------ 1 |2 |43.5 |46.5 2 |3 |44.5 | 3 |4 |45.6 |
您提供的示例数据和输出不会显示左连接。 如果是左连接，则输出将如下所示（请注意，我们对用户1有3个结果，即每个用户1具有的每个增长logging有一次）：
User | age| Data |Growth ------------------------ 1 |2 |43.5 |46.5 1 |2 |43.5 |49.5 1 |2 |43.5 |48.5 2 |3 |44.5 | 3 |4 |45.6 |
假设你仍然需要一个左连接; 这里是你如何做Linq的左连接：
var results = from data in userData join growth in userGrowth on data.User equals growth.User into joined from j in joined.DefaultIfEmpty() select new { UserData = data, UserGrowth = j };
如果你想做一个正确的join，只需要交换你正在select的表格，就像这样：
var results = from growth in userGrowth join data in userData on growth.User equals data.User into joined from j in joined.DefaultIfEmpty() select new { UserData = j, UserGrowth = growth };
代码的重要部分是into语句，后面是DefaultIfEmpty 。 这告诉Linq，如果在另一个表中没有匹配的结果，我们想要有默认值（即null）。
琼斯博士显示了左外连接，但正确的答案会略有不同 – 因为在原来的问题两个表年龄字段链接，以获得结果完全按照需要下面的代码应该使用。
.... //ctx = dataContext class - not shown here. var user1 = new UserData() { User = 1, Age = 2, Data = 43.5 }; var user2 = new UserData() { User = 2, Age = 3, Data = 44.5 }; var user3 = new UserData() { User = 3, Age = 4, Data = 45.6 }; ctx.UserData.AddRange(new List<UserData> { user1, user2, user3 }); var growth1 = new UserGrowth() { Id = 1, Age = 2, Growth = 46.5 }; var growth2 = new UserGrowth() { Id = 1, Age = 5, Growth = 49.5 }; var growth3 = new UserGrowth() { Id = 1, Age = 6, Growth = 48.5 }; ctx.UserGrowth.AddRange(new List<UserGrowth> { growth1, growth2, growth3 }); var query = from userData in ctx.UserData join userGrowth in ctx.UserGrowth on userData.Age equals userGrowth.Age into joinGroup from gr in joinGroup.DefaultIfEmpty() select new { User = userData.User, age = userData.Age, Data = (double?)userData.Data, Growth = (double?)gr.Growth }; Console.WriteLine("{0} | {1} | {2} | {3}", "User", "age", "Data", "Growth"); foreach (var x in query) { Console.WriteLine("{0} | {1} | {2} | {3}", x.User, x.age, x.Data, x.Growth); } .... with following entity classes: public class UserData { [Key] public int User { get; set; } public int Age { get; set; } public double Data { get; set; } } public class UserGrowth { public int Id { get; set; } public int Age { get; set; } public double Growth { get; set; } }
