LINQ Left Join And Right Join

The example data and output you've provided does not demonstrate a left join. If it was a left join your output would look like this (notice how we have 3 results for user 1, i.e. once for each Growth record that user 1 has):

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  |

Assuming that you still require a left join; here's how you do a left join in Linq:

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
              };

If you want to do a right join, just swap the tables that you're selecting from over, like so:

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
              };

The important part of the code is the into statement, followed by the DefaultIfEmpty. This tells Linq that we want to have the default value (i.e. null) if there isn't a matching result in the other table.

Easy way is to use Let keyword. This works for me.

from AItem in Db.A
Let BItem = Db.B.FirstOrDefault(x => x.id == AItem.id ) 
Where SomeCondition
Select new YourViewModel
{
    X1 = AItem.a,
    X2 = AItem.b,
    X3 = BItem.c
}

This is a simulation of Left Join. If each item in B table not match to A item , BItem return null

Doctor Jones showed left outer join, but correct answer would be slightly different - because in the original question two tables linked on age field so to get result exactly as needed following code should be used.

....
//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; }
    }