Calculated Members

Calculated members allow you to define new members on existing dimensions. These members are computed dynamically at query time rather than being stored in the cube. This provides flexibility for scenarios such as creating year-to-date totals, period-over-period comparisons, or custom aggregations.

Understanding Calculated Members

Calculated members are defined using Multidimensional Expressions (MDX). They have a name, an expression that defines their value, and can optionally have formatting properties.

Key Concepts

• MDX Expressions: The core of a calculated member is its MDX expression. This expression can reference existing measures, dimensions, hierarchies, levels, and other calculated members.
• Scope: Calculated members can be defined at the cube level or at the level of a specific dimension. Cube-scoped calculated members are available across all dimensions, while dimension-scoped members are specific to that dimension.
• Parentheses and Order of Operations: MDX expressions follow standard mathematical order of operations, but explicit parentheses are often used to ensure clarity and correctness.
• Set Functions: MDX offers a rich set of set functions (e.g., SUM, AVG, HEAD, TAIL) that are crucial for building complex calculated members.

Creating a Calculated Member

Calculated members can be created using SQL Server Management Studio (SSMS) or programmatically.

Using SQL Server Management Studio (SSMS)

1. Connect to your SQL Server Analysis Services instance.
2. Navigate to your database and cube.
3. Right-click on "Calculated Members" under the "Dimensions" folder (for dimension-scoped) or directly under the "Calculated Members" folder (for cube-scoped).
4. Select "New Calculated Member".
5. In the Calculated Member Editor:
   • Name: Provide a descriptive name for your calculated member.
   • Parent Hierarchy/Level: Specify where the calculated member will reside.
   • MDX Expression: Write your MDX formula.
   • Formatting: Define number formats and other display properties.
6. Click "OK" to save the calculated member.
7. Process the cube for the changes to take effect.

MDX Syntax Example

Here's an example of an MDX expression for a calculated member that calculates the Year-to-Date (YTD) sales:

WITH MEMBER Measures.[YTDSales] AS 
'SUM( YTD( [Date].[Calendar].CurrentMember.Parent.Parent.Parent ), Measures.[SalesAmount] )'
-- The YTD function calculates the period from the beginning of the year to the current member.
-- We navigate up the date hierarchy to find the correct year context.

In this example:

• WITH MEMBER Measures.[YTDSales] AS ... defines a new member named YTDSales within the Measures dimension.
• YTD(...) is an MDX function that returns a set of dates representing the year-to-date period.
• [Date].[Calendar].CurrentMember.Parent.Parent.Parent navigates up the 'Date' dimension's 'Calendar' hierarchy to get to the 'Year' level for context. You might need to adjust this path based on your specific date hierarchy structure.
• Measures.[SalesAmount] refers to an existing measure.

Common Scenarios and Patterns

Period-over-Period Comparisons

Calculating growth or change between two periods.

WITH MEMBER Measures.[SalesGrowthPreviousPeriod] AS 
'IIF( IsEmpty( Measures.[SalesAmount] ) OR IsEmpty( ParallelPeriod( [Date].[Calendar], 1, [Date].[Calendar].CurrentMember ) ), NULL, Measures.[SalesAmount] - Measures.[SalesAmount].Lag(1) )'
-- This calculates the difference between the current period's sales and the previous period's sales.

Budget vs. Actual

Comparing actual performance against budgeted figures.

WITH MEMBER Measures.[VarianceToBudget] AS 
'IIF( IsEmpty( Measures.[ActualSales] ) OR IsEmpty( Measures.[BudgetSales] ), NULL, Measures.[ActualSales] - Measures.[BudgetSales] )'
-- Calculates the difference between actual sales and budget sales.

Best Practices

• Meaningful Names: Use clear and descriptive names for your calculated members.
• Documentation: Add comments within your MDX expressions to explain complex logic.
• Performance: Be mindful of the complexity of your MDX expressions, as poorly written ones can impact query performance. Test thoroughly.
• Scope Appropriateness: Define calculated members at the most restrictive scope possible (dimension-scoped if applicable) to optimize availability and performance.
• Error Handling: Use functions like IsEmpty and IIF to handle potential division-by-zero errors or missing data gracefully.