如何使用 linq 扩展方法执行左外连接

Question

Assuming I have a left outer join as such:

from f in Foo
join b in Bar on f.Foo_Id equals b.Foo_Id into g
from result in g.DefaultIfEmpty()
select new { Foo = f, Bar = result }

How would I express the same task using extension methods? Eg

Foo.GroupJoin(Bar, f => f.Foo_Id, b => b.Foo_Id, (f,b) => ???)
    .Select(???)

Answer 1

For a (left outer) join of a table Bar with a table Foo on Foo.Foo_Id = Bar.Foo_Id in lambda notation:

var qry = Foo.GroupJoin(
          Bar, 
          foo => foo.Foo_Id,
          bar => bar.Foo_Id,
          (x,y) => new { Foo = x, Bars = y })
       .SelectMany(
           x => x.Bars.DefaultIfEmpty(),
           (x,y) => new { Foo=x.Foo, Bar=y});

Answer 2

Since this seems to be the de facto SO question for left outer joins using the method (extension) syntax, I thought I would add an alternative to the currently selected answer that (in my experience at least) has been more commonly what I'm after

// Option 1: Expecting either 0 or 1 matches from the "Right"
// table (Bars in this case):
var qry = Foos.GroupJoin(
          Bars,
          foo => foo.Foo_Id,
          bar => bar.Foo_Id,
          (f,bs) => new { Foo = f, Bar = bs.SingleOrDefault() });

// Option 2: Expecting either 0 or more matches from the "Right" table
// (courtesy of currently selected answer):
var qry = Foos.GroupJoin(
                  Bars, 
                  foo => foo.Foo_Id,
                  bar => bar.Foo_Id,
                  (f,bs) => new { Foo = f, Bars = bs })
              .SelectMany(
                  fooBars => fooBars.Bars.DefaultIfEmpty(),
                  (x,y) => new { Foo = x.Foo, Bar = y });

To display the difference using a simple data set (assuming we're joining on the values themselves):

List<int> tableA = new List<int> { 1, 2, 3 };
List<int?> tableB = new List<int?> { 3, 4, 5 };

// Result using both Option 1 and 2. Option 1 would be a better choice
// if we didn't expect multiple matches in tableB.
{ A = 1, B = null }
{ A = 2, B = null }
{ A = 3, B = 3    }

List<int> tableA = new List<int> { 1, 2, 3 };
List<int?> tableB = new List<int?> { 3, 3, 4 };

// Result using Option 1 would be that an exception gets thrown on
// SingleOrDefault(), but if we use FirstOrDefault() instead to illustrate:
{ A = 1, B = null }
{ A = 2, B = null }
{ A = 3, B = 3    } // Misleading, we had multiple matches.
                    // Which 3 should get selected (not arbitrarily the first)?.

// Result using Option 2:
{ A = 1, B = null }
{ A = 2, B = null }
{ A = 3, B = 3    }
{ A = 3, B = 3    }    

Option 2 is true to the typical left outer join definition, but as I mentioned earlier is often unnecessarily complex depending on the data set.

Answer 3

Group Join method is unnecessary to achieve joining of two data sets.

Inner Join:

var qry = Foos.SelectMany
            (
                foo => Bars.Where (bar => foo.Foo_id == bar.Foo_id),
                (foo, bar) => new
                    {
                    Foo = foo,
                    Bar = bar
                    }
            );

For Left Join just add DefaultIfEmpty()

var qry = Foos.SelectMany
            (
                foo => Bars.Where (bar => foo.Foo_id == bar.Foo_id).DefaultIfEmpty(),
                (foo, bar) => new
                    {
                    Foo = foo,
                    Bar = bar
                    }
            );

EF and LINQ to SQL correctly transform to SQL. For LINQ to Objects it is beter to join using GroupJoin as it internally uses Lookup . But if you are querying DB then skipping of GroupJoin is AFAIK as performant.

Personlay for me this way is more readable compared to GroupJoin().SelectMany()

Answer 4

You can create extension method like:

public static IEnumerable<TResult> LeftOuterJoin<TSource, TInner, TKey, TResult>(this IEnumerable<TSource> source, IEnumerable<TInner> other, Func<TSource, TKey> func, Func<TInner, TKey> innerkey, Func<TSource, TInner, TResult> res)
    {
        return from f in source
               join b in other on func.Invoke(f) equals innerkey.Invoke(b) into g
               from result in g.DefaultIfEmpty()
               select res.Invoke(f, result);
    }

Answer 5

Improving on Ocelot20's answer, if you have a table you're left outer joining with where you just want 0 or 1 rows out of it, but it could have multiple, you need to Order your joined table:

var qry = Foos.GroupJoin(
      Bars.OrderByDescending(b => b.Id),
      foo => foo.Foo_Id,
      bar => bar.Foo_Id,
      (f, bs) => new { Foo = f, Bar = bs.FirstOrDefault() });

Otherwise which row you get in the join is going to be random (or more specifically, whichever the db happens to find first).

Answer 6

Whilst the accepted answer works and is good for Linq to Objects it bugged me that the SQL query isn't just a straight Left Outer Join.

The following code relies on the LinqKit Project that allows you to pass expressions and invoke them to your query.

static IQueryable<TResult> LeftOuterJoin<TSource,TInner, TKey, TResult>(
     this IQueryable<TSource> source, 
     IQueryable<TInner> inner, 
     Expression<Func<TSource,TKey>> sourceKey, 
     Expression<Func<TInner,TKey>> innerKey, 
     Expression<Func<TSource, TInner, TResult>> result
    ) {
    return from a in source.AsExpandable()
            join b in inner on sourceKey.Invoke(a) equals innerKey.Invoke(b) into c
            from d in c.DefaultIfEmpty()
            select result.Invoke(a,d);
}

It can be used as follows

Table1.LeftOuterJoin(Table2, x => x.Key1, x => x.Key2, (x,y) => new { x,y});

Answer 7

Turning Marc Gravell's answer into an extension method, I made the following.

internal static IEnumerable<Tuple<TLeft, TRight>> LeftJoin<TLeft, TRight, TKey>(
    this IEnumerable<TLeft> left,
    IEnumerable<TRight> right,
    Func<TLeft, TKey> selectKeyLeft,
    Func<TRight, TKey> selectKeyRight,
    TRight defaultRight = default(TRight),
    IEqualityComparer<TKey> cmp = null)
{
    return left.GroupJoin(
            right,
            selectKeyLeft,
            selectKeyRight,
            (x, y) => new Tuple<TLeft, IEnumerable<TRight>>(x, y),
            cmp ?? EqualityComparer<TKey>.Default)
        .SelectMany(
            x => x.Item2.DefaultIfEmpty(defaultRight),
            (x, y) => new Tuple<TLeft, TRight>(x.Item1, y));
}

Answer 8

There is an easy solution to this

Just use.HasValue in your Select

.Select(s => new 
{
    FooName = s.Foo_Id.HasValue ? s.Foo.Name : "Default Value"
}

Very easy, no need for groupjoin or anything else

Answer 9

Marc Gravell's answer turn into an extension method that support the IQueryable<T> interface is given in this answer and with added support for C# 8.0 NRT reads as follows:

#nullable enable
using LinqKit;
using LinqKit.Core;
using System.Linq.Expressions;

...

/// <summary>
/// Left join queryable. Linq to SQL compatible. IMPORTANT: any Includes must be put on the source collections before calling this method.
/// </summary>
public static IQueryable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(
    this IQueryable<TOuter> outer,
    IQueryable<TInner> inner,
    Expression<Func<TOuter, TKey>> outerKeySelector,
    Expression<Func<TInner, TKey>> innerKeySelector,
    Expression<Func<TOuter, TInner?, TResult>> resultSelector)
{
    return outer
        .AsExpandable()
        .GroupJoin(
            inner,
            outerKeySelector,
            innerKeySelector,
            (outerItem, innerItems) => new { outerItem, innerItems })
        .SelectMany(
            joinResult => joinResult.innerItems.DefaultIfEmpty(),
            (joinResult, innerItem) =>
                resultSelector.Invoke(joinResult.outerItem, innerItem));
}

Answer 10

I have this question bookmarked and need to reference it every year or so. Each time I revisit this, I find I have forgotten how it works. Here's a more detailed explanation of what's happening.

GroupJoin is like a mix of GroupBy and Join . GroupJoin basically groups the outer collection by the join key, then joins the groupings to the inner collection on the join key. Suppose we have customers and orders. If you GroupJoin on the respective IDs, the result is an enumerable of {Customer, IGrouping<int, Order>} . The reason GroupJoin is useful is because all inner objects are represented even if the outer collection contains no matching objects. For customers with no orders, the IGrouping<int, Order> is simply empty. Once we have { Customer, IGrouping<int, Order> } , we can use as-is, filter out results that have no orders, or flatten with SelectMany to get results like a traditional LINQ Join .

Here's a full example if anyone wants to step through with the debugger and see how this works:

using System;
using System.Linq;
                    
public class Program
{
    public static void Main()
    {
        //Create some customers
        var customers = new Customer[]
        {
            new Customer(1, "Alice"),
            new Customer(2, "Bob"),
            new Customer(3, "Carol")
        };
        
        //Create some orders for Alice and Bob, but none for Carol
        var orders = new Order[]
        {
            new Order(1, 1),
            new Order(2, 1),
            new Order(3, 1),
            new Order(4, 2),
            new Order(5, 2)
        };

        //Group join customers to orders.
        //Result is IEnumerable<Customer, IGrouping<int, Order>>. 
        //Every customer will be present. 
        //If a customer has no orders, the IGrouping<> will be empty.
        var groupJoined = customers.GroupJoin(orders,
                              c => c.ID,
                              o => o.CustomerID,
                              (customer, order) => (customer, order));

        //Display results. Prints:
        //    Customer: Alice (CustomerID=1), Orders: 3
        //    Customer: Bob (CustomerID=2), Orders: 2
        //    Customer: Carol (CustomerID=3), Orders: 0
        foreach(var result in groupJoined)
        {
            Console.WriteLine($"Customer: {result.customer.Name} (CustomerID={result.customer.ID}), Orders: {result.order.Count()}");
        }
        
        //Flatten the results to look more like a LINQ join
        //Produces an enumerable of { Customer, Order }
        //All customers represented, order is null if customer has no orders
        var flattened = groupJoined.SelectMany(z => z.order.DefaultIfEmpty().Select(y => new { z.customer, y }));

        //Get only results where the outer table is null.
        //roughly equivalent to: 
        //SELECT * 
        //FROM A 
        //LEFT JOIN B 
        //ON A.ID = B.ID 
        //WHERE B.ID IS NULL;
        var noMatch = groupJoined.Where(z => z.order.DefaultIfEmpty().Count() == 0);
    }
}

class Customer
{
    public int ID { get; set; }
    public string Name { get; set; }

    public Customer(int iD, string name)
    {
        ID = iD;
        Name = name;
    }
}

class Order
{
    static Random Random { get; set; } = new Random();

    public int ID { get; set; }
    public int CustomerID { get; set; }
    public decimal Amount { get; set; }

    public Order(int iD, int customerID)
    {
        ID = iD;
        CustomerID = customerID;
        Amount = (decimal)Random.Next(1000, 10000) / 100;
    }
}