RowNumber Function (Reporting Services)
Returns a running count of the number of rows for the specified scope.
RowNumber returns a running value of the count of rows within the specified scope, just as RunningValue returns the running value of an aggregate function. When you specify a scope, you specify when to reset the row count to 1.
scope cannot be an expression. scope must be a containing scope. Typical scopes, from the outermost to the innermost containment, are report dataset, data region, row groups or column groups.
To increment values across columns, specify a scope that is the name of a column group. To increment numbers down rows, specify a scope that is the name of a row group.
Including aggregates that specify both a row group and a column group in a single expression is not supported.
The following is an expression that you can use for the BackgroundColor property of a Tablix data region detail row to alternate the color of detail rows for each group, always beginning with White..
=IIF(RowNumber("GroupbyCategory") Mod 2, "White", "PaleGreen")