Some custom aggregate translations

* string.Join (SQL Server and SQLite)
* string.Concat (SQL Server and SQLite)
* Standard deviation and variance (SQL Server)

Closes dotnet#2981
Closes dotnet#28104

src/EFCore.Relational/Query/Internal/QueryableAggregateMethodTranslator.cs

@@ -74,6 +74,8 @@ public QueryableAggregateMethodTranslator(ISqlExpressionFactory sqlExpressionFac
                             averageSqlExpression.Type,
                             averageSqlExpression.TypeMapping);
 
+                // Count/LongCount are special since if the argument is a star fragment, it needs to be transformed to any non-null constant
+                // when a predicate is applied.
                 case nameof(Queryable.Count)
                 when methodInfo == QueryableMethods.CountWithoutPredicate
                     || methodInfo == QueryableMethods.CountWithPredicate:

src/EFCore.SqlServer/Query/Internal/SqlServerAggregateMethodCallTranslatorProvider.cs

@@ -21,10 +21,14 @@ public SqlServerAggregateMethodCallTranslatorProvider(RelationalAggregateMethodC
         : base(dependencies)
     {
         var sqlExpressionFactory = dependencies.SqlExpressionFactory;
+        var typeMappingSource = dependencies.RelationalTypeMappingSource;
+
         AddTranslators(
             new IAggregateMethodCallTranslator[]
             {
-                new SqlServerLongCountMethodTranslator(sqlExpressionFactory)
+                new SqlServerLongCountMethodTranslator(sqlExpressionFactory),
+                new SqlServerStatisticsAggregateMethodTranslator(sqlExpressionFactory, typeMappingSource),
+                new SqlServerStringAggregateMethodTranslator(sqlExpressionFactory, typeMappingSource)
             });
     }
 }

src/EFCore.SqlServer/Query/Internal/SqlServerExpression.cs

@@ -0,0 +1,104 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.EntityFrameworkCore.Query.SqlExpressions;
+
+namespace Microsoft.EntityFrameworkCore.SqlServer.Query.Internal;
+
+/// <summary>
+///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+///     any release. You should only use it directly in your code with extreme caution and knowing that
+///     doing so can result in application failures when updating to a new Entity Framework Core release.
+/// </summary>
+public static class SqlServerExpression
+{
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    public static SqlFunctionExpression AggregateFunction(
+        ISqlExpressionFactory sqlExpressionFactory,
+        string name,
+        IEnumerable<SqlExpression> arguments,
+        EnumerableExpression enumerableExpression,
+        int enumerableArgumentIndex,
+        bool nullable,
+        IEnumerable<bool> argumentsPropagateNullability,
+        Type returnType,
+        RelationalTypeMapping? typeMapping = null)
+        => new(
+            name,
+            ProcessAggregateFunctionArguments(sqlExpressionFactory, arguments, enumerableExpression, enumerableArgumentIndex),
+            nullable,
+            argumentsPropagateNullability,
+            returnType,
+            typeMapping);
+
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    public static SqlFunctionExpression AggregateFunctionWithOrdering(
+        ISqlExpressionFactory sqlExpressionFactory,
+        string name,
+        IEnumerable<SqlExpression> arguments,
+        EnumerableExpression enumerableExpression,
+        int enumerableArgumentIndex,
+        bool nullable,
+        IEnumerable<bool> argumentsPropagateNullability,
+        Type returnType,
+        RelationalTypeMapping? typeMapping = null)
+        => enumerableExpression.Orderings.Count == 0
+            ? AggregateFunction(sqlExpressionFactory, name, arguments, enumerableExpression, enumerableArgumentIndex, nullable, argumentsPropagateNullability, returnType, typeMapping)
+            : new SqlServerSqlFunctionExpression(
+                name,
+                ProcessAggregateFunctionArguments(sqlExpressionFactory, arguments, enumerableExpression, enumerableArgumentIndex),
+                enumerableExpression.Orderings,
+                nullable,
+                argumentsPropagateNullability,
+                returnType,
+                typeMapping);
+
+    private static IReadOnlyList<SqlExpression> ProcessAggregateFunctionArguments(
+        ISqlExpressionFactory sqlExpressionFactory,
+        IEnumerable<SqlExpression> arguments,
+        EnumerableExpression enumerableExpression,
+        int enumerableArgumentIndex)
+    {
+        var argIndex = 0;
+        var typeMappedArguments = new List<SqlExpression>();
+
+        foreach (var argument in arguments)
+        {
+            var modifiedArgument = sqlExpressionFactory.ApplyDefaultTypeMapping(argument);
+
+            if (argIndex == enumerableArgumentIndex)
+            {
+                // This is the argument representing the enumerable inputs to be aggregated.
+                // Wrap it with a CASE/WHEN for the predicate and with DISTINCT, if necessary.
+                if (enumerableExpression.Predicate != null)
+                {
+                    modifiedArgument = sqlExpressionFactory.Case(
+                        new List<CaseWhenClause> { new(enumerableExpression.Predicate, modifiedArgument) },
+                        elseResult: null);
+                }
+
+                if (enumerableExpression.IsDistinct)
+                {
+                    modifiedArgument = new DistinctExpression(modifiedArgument);
+                }
+            }
+
+            typeMappedArguments.Add(modifiedArgument);
+
+            argIndex++;
+        }
+
+        return typeMappedArguments;
+    }
+}

src/EFCore.SqlServer/Query/Internal/SqlServerParameterBasedSqlProcessor.cs

@@ -46,4 +46,14 @@ public class SqlServerParameterBasedSqlProcessor : RelationalParameterBasedSqlPr
 
         return new SearchConditionConvertingExpressionVisitor(Dependencies.SqlExpressionFactory).Visit(optimizedQueryExpression);
     }
+
+    /// <inheritdoc />
+    protected override Expression ProcessSqlNullability(
+        Expression selectExpression, IReadOnlyDictionary<string, object?> parametersValues, out bool canCache)
+    {
+        Check.NotNull(selectExpression, nameof(selectExpression));
+        Check.NotNull(parametersValues, nameof(parametersValues));
+
+        return new SqlServerSqlNullabilityProcessor(Dependencies, UseRelationalNulls).Process(selectExpression, parametersValues, out canCache);
+    }
 }

src/EFCore.SqlServer/Query/Internal/SqlServerQuerySqlGenerator.cs

@@ -95,6 +95,37 @@ protected override void GenerateLimitOffset(SelectExpression selectExpression)
         }
     }
 
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    protected override Expression VisitSqlFunction(SqlFunctionExpression sqlFunctionExpression)
+    {
+        base.VisitSqlFunction(sqlFunctionExpression);
+
+        if (sqlFunctionExpression is SqlServerSqlFunctionExpression sqlServerFunctionExpression
+            && sqlServerFunctionExpression.AggregateOrderings.Count > 0)
+        {
+            Sql.Append(" WITHIN GROUP (ORDER BY ");
+
+            for (var i = 0; i < sqlServerFunctionExpression.AggregateOrderings.Count; i++)
+            {
+                if (i > 0)
+                {
+                    Sql.Append(", ");
+                }
+
+                Visit(sqlServerFunctionExpression.AggregateOrderings[i]);
+            }
+
+            Sql.Append(")");
+        }
+
+        return sqlFunctionExpression;
+    }
+
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
     ///     the same compatibility standards as public APIs. It may be changed or removed without notice in

src/EFCore.SqlServer/Query/Internal/SqlServerSqlExpressionFactory.cs

@@ -14,7 +14,7 @@ namespace Microsoft.EntityFrameworkCore.SqlServer.Query.Internal;
 /// </summary>
 public class SqlServerSqlExpressionFactory : SqlExpressionFactory
 {
-    private IRelationalTypeMappingSource _typeMappingSource;
+    private readonly IRelationalTypeMappingSource _typeMappingSource;
 
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to

src/EFCore.SqlServer/Query/Internal/SqlServerSqlFunctionExpression.cs

@@ -0,0 +1,165 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.EntityFrameworkCore.Query.SqlExpressions;
+
+namespace Microsoft.EntityFrameworkCore.SqlServer.Query.Internal;
+
+/// <summary>
+///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+///     any release. You should only use it directly in your code with extreme caution and knowing that
+///     doing so can result in application failures when updating to a new Entity Framework Core release.
+/// </summary>
+public class SqlServerSqlFunctionExpression : SqlFunctionExpression, IEquatable<SqlServerSqlFunctionExpression>
+{
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    public SqlServerSqlFunctionExpression(
+        string functionName,
+        IEnumerable<SqlExpression> arguments,
+        IReadOnlyList<OrderingExpression> aggregateOrderings,
+        bool nullable,
+        IEnumerable<bool> argumentsPropagateNullability,
+        Type type,
+        RelationalTypeMapping? typeMapping)
+        : base(functionName, arguments, nullable, argumentsPropagateNullability, type, typeMapping)
+        => AggregateOrderings = aggregateOrderings;
+
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    public virtual IReadOnlyList<OrderingExpression> AggregateOrderings { get; }
+
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    protected override Expression VisitChildren(ExpressionVisitor visitor)
+    {
+        var visitedBase = (SqlFunctionExpression)base.VisitChildren(visitor);
+
+        OrderingExpression[]? visitedAggregateOrderings = null;
+
+        for (var i = 0; i < AggregateOrderings.Count; i++)
+        {
+            var visitedOrdering = (OrderingExpression)visitor.Visit(AggregateOrderings[i]);
+            if (visitedOrdering != AggregateOrderings[i] && visitedAggregateOrderings is null)
+            {
+                visitedAggregateOrderings = new OrderingExpression[AggregateOrderings.Count];
+
+                for (var j = 0; j < visitedAggregateOrderings.Length; j++)
+                {
+                    visitedAggregateOrderings[j] = AggregateOrderings[j];
+                }
+            }
+
+            if (visitedAggregateOrderings is not null)
+            {
+                visitedAggregateOrderings[i] = visitedOrdering;
+            }
+        }
+
+        return visitedBase != this || visitedAggregateOrderings is not null
+            ? new SqlServerSqlFunctionExpression(
+                Name,
+                visitedBase.Arguments!,
+                visitedAggregateOrderings ?? AggregateOrderings,
+                IsNullable,
+                ArgumentsPropagateNullability!,
+                Type,
+                TypeMapping)
+            : this;
+    }
+
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    public override SqlServerSqlFunctionExpression ApplyTypeMapping(RelationalTypeMapping? typeMapping)
+        => new(
+            Name,
+            Arguments!,
+            AggregateOrderings,
+            IsNullable,
+            ArgumentsPropagateNullability!,
+            Type,
+            typeMapping ?? TypeMapping);
+
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    public override SqlFunctionExpression Update(SqlExpression? instance, IReadOnlyList<SqlExpression>? arguments)
+    {
+        Check.DebugAssert(arguments is not null, "arguments is not null");
+        Check.DebugAssert(instance is null, "instance not supported on SqlServerFunctionExpression");
+
+        return arguments.SequenceEqual(Arguments!)
+            ? this
+            : new SqlServerSqlFunctionExpression(
+                Name, arguments, AggregateOrderings, IsNullable, ArgumentsPropagateNullability!, Type, TypeMapping);
+    }
+
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    public virtual SqlFunctionExpression UpdateAggregateOrderings(IReadOnlyList<OrderingExpression> aggregateOrderings)
+        => aggregateOrderings.SequenceEqual(AggregateOrderings)
+            ? this
+            : new SqlServerSqlFunctionExpression(
+                Name, Arguments!, aggregateOrderings, IsNullable, ArgumentsPropagateNullability!, Type, TypeMapping);
+
+    /// <inheritdoc />
+    protected override void Print(ExpressionPrinter expressionPrinter)
+    {
+        base.Print(expressionPrinter);
+
+        if (AggregateOrderings.Count > 0)
+        {
+            expressionPrinter.Append(" WITHIN GROUP (ORDER BY ");
+            expressionPrinter.VisitCollection(AggregateOrderings);
+            expressionPrinter.Append(")");
+        }
+    }
+
+    /// <inheritdoc />
+    public override bool Equals(object? obj)
+        => obj is SqlServerSqlFunctionExpression sqlServerFunctionExpression && Equals(sqlServerFunctionExpression);
+
+    /// <inheritdoc />
+    public virtual bool Equals(SqlServerSqlFunctionExpression? other)
+        => ReferenceEquals(this, other)
+            || base.Equals(other) && AggregateOrderings.SequenceEqual(other.AggregateOrderings);
+
+    /// <inheritdoc />
+    public override int GetHashCode()
+    {
+        var hash = new HashCode();
+
+        hash.Add(base.GetHashCode());
+
+        foreach (var orderingExpression in AggregateOrderings)
+        {
+            hash.Add(orderingExpression);
+        }
+
+        return hash.ToHashCode();
+    }
+}