/// <summary>
        /// Repackages and passes an expression from an aggregate subquery back to the parent query.
        /// </summary>
        private SqlExpression CreateAggregateMappingExpression(SqlQuery parentQuery, SqlTable proxyTable, SqlExpression sqlExpr)
        {
            var subQuery = proxyTable.SubQuery.Queries[0];

            string alias         = parentQuery.AliasManager.CreateAlias("aggCol");
            var    groupByColumn = new SqlSelectItem
            {
                Expression = sqlExpr,
                Alias      = alias
            };

            subQuery.SelectClause.Items.Add(groupByColumn);
            subQuery.GroupByClause.Expressions.Add(groupByColumn.Expression);

            // Register an expression that can be used to access the group-by expression
            string mappedSql     = GetColumnSql(proxyTable, alias);
            var    mappedSqlExpr = new SqlExpression(mappedSql);

            SqlExpression.CopyTransforms(sqlExpr, mappedSqlExpr);
            mappedSqlExpr.DatabaseType = sqlExpr.DatabaseType;
            return(mappedSqlExpr);
        }