--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Source/Data/Linq/Builder/AggregationBuilder.cs	Thu Mar 27 21:46:09 2014 +0400
@@ -0,0 +1,168 @@
+using System;
+using System.Linq;
+using System.Linq.Expressions;
+
+namespace BLToolkit.Data.Linq.Builder
+{
+	using BLToolkit.Linq;
+	using Data.Sql;
+	using Reflection;
+
+	class AggregationBuilder : MethodCallBuilder
+	{
+		public static string[] MethodNames = new[] { "Average", "Min", "Max", "Sum" };
+
+		protected override bool CanBuildMethodCall(ExpressionBuilder builder, MethodCallExpression methodCall, BuildInfo buildInfo)
+		{
+			return methodCall.IsQueryable(MethodNames);
+		}
+
+		protected override IBuildContext BuildMethodCall(ExpressionBuilder builder, MethodCallExpression methodCall, BuildInfo buildInfo)
+		{
+			var sequence = builder.BuildSequence(new BuildInfo(buildInfo, methodCall.Arguments[0]));
+
+			if (sequence.SqlQuery.Select.IsDistinct        ||
+			    sequence.SqlQuery.Select.TakeValue != null ||
+			    sequence.SqlQuery.Select.SkipValue != null ||
+			   !sequence.SqlQuery.GroupBy.IsEmpty)
+			{
+				sequence = new SubQueryContext(sequence);
+			}
+
+			if (!sequence.SqlQuery.OrderBy.IsEmpty)
+			{
+				if (sequence.SqlQuery.Select.TakeValue == null && sequence.SqlQuery.Select.SkipValue == null)
+					sequence.SqlQuery.OrderBy.Items.Clear();
+				else
+					sequence = new SubQueryContext(sequence);
+			}
+
+			var context = new AggregationContext(buildInfo.Parent, sequence, methodCall);
+			var sql     = sequence.ConvertToSql(null, 0, ConvertFlags.Field).Select(_ => _.Sql).ToArray();
+
+			if (sql.Length == 1 && sql[0] is SqlQuery)
+			{
+				var query = (SqlQuery)sql[0];
+
+				if (query.Select.Columns.Count == 1)
+				{
+					var join = SqlQuery.OuterApply(query);
+					context.SqlQuery.From.Tables[0].Joins.Add(join.JoinedTable);
+					sql[0] = query.Select.Columns[0];
+				}
+			}
+
+			context.Sql        = context.SqlQuery;
+			context.FieldIndex = context.SqlQuery.Select.Add(
+				new SqlFunction(methodCall.Type, methodCall.Method.Name, sql));
+
+			return context;
+		}
+
+		protected override SequenceConvertInfo Convert(
+			ExpressionBuilder builder, MethodCallExpression methodCall, BuildInfo buildInfo, ParameterExpression param)
+		{
+			return null;
+		}
+
+		class AggregationContext : SequenceContextBase
+		{
+			public AggregationContext(IBuildContext parent, IBuildContext sequence, MethodCallExpression methodCall)
+				: base(parent, sequence, null)
+			{
+				_returnType = methodCall.Method.ReturnType;
+				_methodName = methodCall.Method.Name;
+			}
+
+			readonly string _methodName;
+			readonly Type   _returnType;
+			private  SqlInfo[] _index;
+
+			public int            FieldIndex;
+			public ISqlExpression Sql;
+
+			static object CheckNullValue(object value, object context)
+			{
+				if (value == null || value is DBNull)
+					throw new InvalidOperationException(string.Format("Function {0} returns non-nullable value, but result is NULL. Use nullable version of the function instead.", context));
+
+				return value;
+			}
+
+			public override void BuildQuery<T>(Query<T> query, ParameterExpression queryParameter)
+			{
+				var expr   = BuildExpression(FieldIndex);
+				var mapper = Builder.BuildMapper<object>(expr);
+
+				query.SetElementQuery(mapper.Compile());
+			}
+
+			public override Expression BuildExpression(Expression expression, int level)
+			{
+				return BuildExpression(ConvertToIndex(expression, level, ConvertFlags.Field)[0].Index);
+			}
+
+			Expression BuildExpression(int fieldIndex)
+			{
+				Expression expr;
+
+				if (_returnType.IsClass || _methodName == "Sum" || TypeHelper.IsNullableType(_returnType))
+				{
+					expr = Builder.BuildSql(_returnType, fieldIndex);
+				}
+				else
+				{
+					expr = Builder.BuildSql(
+						_returnType,
+						fieldIndex, 
+						ReflectionHelper.Expressor<object>.MethodExpressor(o => CheckNullValue(o, o)),
+						Expression.Constant(_methodName));
+				}
+
+				return expr;
+			}
+
+			public override SqlInfo[] ConvertToSql(Expression expression, int level, ConvertFlags flags)
+			{
+				switch (flags)
+				{
+					case ConvertFlags.All   :
+					case ConvertFlags.Key   :
+					case ConvertFlags.Field : return Sequence.ConvertToSql(expression, level + 1, flags);
+				}
+
+				throw new InvalidOperationException();
+			}
+
+			public override SqlInfo[] ConvertToIndex(Expression expression, int level, ConvertFlags flags)
+			{
+				switch (flags)
+				{
+					case ConvertFlags.Field :
+						return _index ?? (_index = new[]
+						{
+							new SqlInfo { Query = Parent.SqlQuery, Index = Parent.SqlQuery.Select.Add(Sql), Sql = Sql, }
+						});
+				}
+
+				throw new InvalidOperationException();
+			}
+
+			public override IsExpressionResult IsExpression(Expression expression, int level, RequestFor requestFlag)
+			{
+				switch (requestFlag)
+				{
+					case RequestFor.Root       : return new IsExpressionResult(Lambda != null && expression == Lambda.Parameters[0]);
+					case RequestFor.Expression : return IsExpressionResult.True;
+				}
+
+				return IsExpressionResult.False;
+			}
+
+			public override IBuildContext GetContext(Expression expression, int level, BuildInfo buildInfo)
+			{
+				throw new InvalidOperationException();
+			}
+		}
+	}
+}