The total and subtotal rows created by ROLLUP and CUBE contain placeholder NULLs in any column specified in the SELECT list that was not used for the grouping. This means that when you are examining your results, you cannot distinguish whether a NULL in a subtotal row is a placeholder NULL, or a NULL resulting from the evaluation of the underlying data for the row. As a consequence, it is also difficult to distinguish between a detail row, a subtotal row, and a grand total row.

The GROUPING function allows you to distinguish placeholder NULLs from NULLs caused by underlying data. If you specify a GROUPING function with one group-by-expression from the grouping set specification, the function returns a 1 if it is a placeholder NULL, and 0 if it reflects a value (perhaps NULL) present in the underlying data for that row.

For example, the following query returns the result set shown in the table below:

SELECT Employees.EmployeeID AS Employee,
   YEAR( OrderDate ) AS Year,
   COUNT( SalesOrders.ID ) AS Orders,
   GROUPING( Employee ) AS GE,
   GROUPING( Year ) AS GY
FROM Employees LEFT OUTER JOIN SalesOrders
   ON Employees.EmployeeID = SalesOrders.SalesRepresentative
WHERE Employees.Sex IN ( 'F' )
   AND Employees.State IN ( 'TX' , 'NY' )
GROUP BY GROUPING SETS ( ( Year, Employee ), ( Year ), ( ) )
ORDER BY Year, Employee;

This query returns the following results:

In this example, row 1 represents the grand total of orders (54) because the empty grouping set '()' was specified. Notice that GE and GY both contain a 1 to indicate that the NULLs in the Employees and Year columns are placeholder NULLs for Employees and Year columns, respectively.

Row 2 is a subtotal row. The 1 in the GE column indicates that the NULL in the Employees column is a placeholder NULL. The 0 in the GY column indicates that the NULL in the Year column is the result of evaluating the underlying data, and not a placeholder NULL; in this case, this row represents those employees who have no orders.

Rows 3-7 show the total number of orders, per employee, where the Year was NULL. That is, these are the female employees that live in Texas and New York who have no orders. These are the detail rows for row 2. That is, row 2 is a totaling of rows 3-7.

Row 8 is a subtotal row showing the number of orders for all employees combined, in the year 2000. Row 9 is the single detail row for row 8.

Row 10 is a subtotal row showing the number of orders for all employees combined, in the year 2001. Row 11 is the single detail row for row 10.

For more information about the syntax of the GROUPING function, see GROUPING function [Aggregate].