private Field SelectAggregate(string functionName, params SqlExpression[] args)
{
var column = new SelectColumn(SqlExpression.FunctionCall(functionName, args));
var query = new SqlQueryExpression(new[] { column });
query.FromClause.AddTable("APP.test_table");
var result = AdminQuery.Select(query);
var row = result.FirstOrDefault();
if (row == null)
throw new InvalidOperationException();
return row.GetValue(0);
}