Fix to #29667 - Count after Take throws "No column name was specified…

… for column 1 of 't'."

Problem was that in some cases (e.g. count over keyless entity that has been pushed down) we now generate empty projection in the subquery, where before we were projecting some columns.
SQL Server doesn't allow unaliased projection in the subquery, so the fix is to simply add an alias to the empty projection that we generate.

Fixes #29667

src/EFCore.Relational/Query/QuerySqlGenerator.cs

@@ -248,6 +248,10 @@ protected override Expression VisitSelect(SelectExpression selectExpression)
             else
             {
                 _relationalCommandBuilder.Append("1");
+                if (selectExpression.Alias != null)
+                {
+                    _relationalCommandBuilder.Append(" AS empty");
+                }
             }
 
             if (selectExpression.Tables.Any())

test/EFCore.Relational.Specification.Tests/Query/KeylessEntitiesQueryAdHocTestBase.cs

@@ -0,0 +1,90 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.EntityFrameworkCore.Query;
+
+public abstract class KeylessEntitiesQueryAdHocTestBase : NonSharedModelTestBase
+{
+    protected KeylessEntitiesQueryAdHocTestBase(ITestOutputHelper testOutputHelper)
+    {
+        //TestSqlLoggerFactory.SetTestOutputHelper(testOutputHelper);
+    }
+
+    protected override string StoreName
+        => "KeylessEntitiesQueryAdHocTest";
+
+    #region 29667
+
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual async Task Count_over_keyless_entities(bool async)
+    {
+        var contextFactory = await Initialize29667();
+        using var context = contextFactory.CreateContext();
+        var result = async
+            ? await context.KeylessEntities.CountAsync()
+            : context.KeylessEntities.Count();
+
+        Assert.Equal(5, result);
+    }
+
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual async Task Count_over_keyless_entities_with_pushdown(bool async)
+    {
+        var contextFactory = await Initialize29667();
+        using var context = contextFactory.CreateContext();
+
+        var query = context.KeylessEntities.OrderBy(x => x.Number).Take(10);
+        var result = async
+            ? await query.CountAsync()
+            : query.Count();
+
+        Assert.Equal(5, result);
+    }
+
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual async Task Count_over_keyless_entities_with_pushdown_empty_projection(bool async)
+    {
+        var contextFactory = await Initialize29667();
+        using var context = contextFactory.CreateContext();
+
+        var query = context.KeylessEntities.Take(10);
+        var result = async
+            ? await query.CountAsync()
+            : query.Count();
+
+        Assert.Equal(5, result);
+    }
+
+    protected abstract void Seed29667(MyContext29667 ctx);
+
+    protected Task<ContextFactory<MyContext29667>> Initialize29667()
+        => InitializeAsync<MyContext29667>(
+            seed: Seed29667,
+            onConfiguring: ob => ob.ConfigureWarnings(x => x.Ignore(CoreEventId.RowLimitingOperationWithoutOrderByWarning)));
+
+    protected class MyContext29667 : DbContext
+    {
+        public MyContext29667(DbContextOptions options)
+            : base(options)
+        {
+        }
+
+        public DbSet<MyKeylessEntity29667> KeylessEntities { get; set; }
+
+        protected override void OnModelCreating(ModelBuilder modelBuilder)
+        {
+            modelBuilder.Entity<MyKeylessEntity29667>().HasNoKey();
+        }
+    }
+
+    public class MyKeylessEntity29667
+    {
+        public string Name { get; set; }
+        public int? Number { get; set; }
+    }
+
+    #endregion
+}

test/EFCore.SqlServer.FunctionalTests/Query/KeylessEntitiesQueryAdHocSqlServerTest.cs

@@ -0,0 +1,68 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.EntityFrameworkCore.Query;
+
+public class KeylessEntitiesQueryAdHocSqlServerTest : KeylessEntitiesQueryAdHocTestBase
+{
+    public KeylessEntitiesQueryAdHocSqlServerTest(ITestOutputHelper testOutputHelper)
+        : base(testOutputHelper)
+    {
+    }
+
+    protected override ITestStoreFactory TestStoreFactory
+        => SqlServerTestStoreFactory.Instance;
+
+    public override async Task Count_over_keyless_entities(bool async)
+    {
+        await base.Count_over_keyless_entities(async);
+
+        AssertSql(
+"""
+SELECT COUNT(*)
+FROM [KeylessEntities] AS [k]
+""");
+    }
+
+    public override async Task Count_over_keyless_entities_with_pushdown(bool async)
+    {
+        await base.Count_over_keyless_entities_with_pushdown(async);
+
+        AssertSql(
+"""
+@__p_0='10'
+
+SELECT COUNT(*)
+FROM (
+    SELECT TOP(@__p_0) [k].[Number]
+    FROM [KeylessEntities] AS [k]
+    ORDER BY [k].[Number]
+) AS [t]
+""");
+    }
+
+    public override async Task Count_over_keyless_entities_with_pushdown_empty_projection(bool async)
+    {
+        await base.Count_over_keyless_entities_with_pushdown_empty_projection(async);
+
+        AssertSql(
+"""
+@__p_0='10'
+
+SELECT COUNT(*)
+FROM (
+    SELECT TOP(@__p_0) 1 AS empty
+    FROM [KeylessEntities] AS [k]
+) AS [t]
+""");
+    }
+
+    protected override void Seed29667(MyContext29667 ctx)
+    {
+        ctx.Database.ExecuteSqlRaw(@"INSERT INTO [KeylessEntities] ([Name], [Number]) VALUES
+(N'k1', 1), (N'k1', 2), (N'k1', 3), (N'k2', 1), (N'k2', 1)");
+    }
+
+    private void AssertSql(params string[] expected)
+        => ((TestSqlLoggerFactory)ServiceProvider.GetRequiredService<ILoggerFactory>()).AssertBaseline(expected);
+}

test/EFCore.Sqlite.FunctionalTests/Query/KeylessEntitiesQueryAdHocSqliteTest.cs

@@ -0,0 +1,74 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.EntityFrameworkCore.Query;
+
+public class KeylessEntitiesQueryAdHocSqliteTest : KeylessEntitiesQueryAdHocTestBase
+{
+    public KeylessEntitiesQueryAdHocSqliteTest(ITestOutputHelper testOutputHelper)
+        : base(testOutputHelper)
+    {
+    }
+
+    protected override ITestStoreFactory TestStoreFactory
+        => SqliteTestStoreFactory.Instance;
+
+    public override async Task Count_over_keyless_entities(bool async)
+    {
+        await base.Count_over_keyless_entities(async);
+
+        AssertSql(
+"""
+SELECT COUNT(*)
+FROM "KeylessEntities" AS "k"
+""");
+    }
+
+    public override async Task Count_over_keyless_entities_with_pushdown(bool async)
+    {
+        await base.Count_over_keyless_entities_with_pushdown(async);
+
+        AssertSql(
+"""
+@__p_0='10'
+
+SELECT COUNT(*)
+FROM (
+    SELECT "k"."Number"
+    FROM "KeylessEntities" AS "k"
+    ORDER BY "k"."Number"
+    LIMIT @__p_0
+) AS "t"
+""");
+    }
+
+    public override async Task Count_over_keyless_entities_with_pushdown_empty_projection(bool async)
+    {
+        await base.Count_over_keyless_entities_with_pushdown_empty_projection(async);
+
+        AssertSql(
+"""
+@__p_0='10'
+
+SELECT COUNT(*)
+FROM (
+    SELECT 1 AS empty
+    FROM "KeylessEntities" AS "k"
+    LIMIT @__p_0
+) AS "t"
+""");
+    }
+
+    protected override void Seed29667(MyContext29667 ctx)
+    {
+        ctx.Database.ExecuteSqlRaw(@"INSERT INTO [KeylessEntities] ([Name], [Number]) VALUES
+('k1', 1), ('k1', 2), ('k1', 3), ('k2', 1), ('k2', 1)");
+        //ctx.Database.ExecuteSqlRaw(@"INSERT INTO [KeylessEntities] ([Name], [Number]) VALUES(N'k1', 2)");
+        //ctx.Database.ExecuteSqlRaw(@"INSERT INTO [KeylessEntities] ([Name], [Number]) VALUES(N'k1', 3)");
+        //ctx.Database.ExecuteSqlRaw(@"INSERT INTO [KeylessEntities] ([Name], [Number]) VALUES(N'k2', 1)");
+        //ctx.Database.ExecuteSqlRaw(@"INSERT INTO [KeylessEntities] ([Name], [Number]) VALUES(N'k2', 2)");
+    }
+
+    private void AssertSql(params string[] expected)
+        => ((TestSqlLoggerFactory)ServiceProvider.GetRequiredService<ILoggerFactory>()).AssertBaseline(expected);
+}