1. Fun with Scoped Assignments
Chris Webb

2. Who Am I? Chris Webb chris@crossjoin.co.uk
Independent Analysis Services and MDX consultant and trainer
Author: “MDX Solutions” and “Expert Cube Development with SSAS 2008”
SQL Server MVP
Blogger:

3. Agenda What are scoped assignments and why should I use them?
Defining subcubes
Assignments to calculated measures
Complex subcubes
Assignments to real measures and aggregation
FREEZE and IF
Assigning to properties

4. What are scoped assignments?
Scoped assignments allow you to overwrite the value of cells inside your cube
Contrast this with calculated members, which extend the space of the cube
They are defined on the MDX Script of your cube, on the Calculations tab in BIDS

5. Calculated Members

6. Calculated Members

7. Scoped Assignments

8. Scoped Assignments

9. Why should I use them? Among other things, they allow you to:
Make calculations behave in different ways in different parts of the cube
Break up logic for complex calculations into more comprehensible parts
Perform calculations at low levels of granularity that can then be aggregated up very efficiently
They are often the most efficient way of writing certain calculations

10. Defining subcubes Scoped assignments have two parts:
The subcube definition - the part of the cube whose value will be overwritten
The assignment - the new value written to the subcube
There are two ways of defining a subcube
Direct assignment
The SCOPE statement

11. Direct assignment syntax
Direct assignment uses the following syntax: (subcube definition) = value;
For example: ([Measures].[Internet Sales Amount])=1;
I prefer not to use it, because this syntax can very quickly become unreadable

12. SCOPE statements
SCOPE statements take the form: SCOPE(subcube definition); THIS = value; END SCOPE;
For example: SCOPE([Measures].[Internet Sales Amount]); THIS=1; END SCOPE;

13. Nesting SCOPE statements
The advantage of using SCOPE statements is that you can nest several to build up complex subcubes, eg: SCOPE([Measures].[Internet Sales Amount]); SCOPE([Date].[Calendar Year].[Calendar Year].MEMBERS); THIS=1; END SCOPE; END SCOPE;
This has obvious benefits for readability
You can also make assignments at different levels within the nesting

14. Subcube definition rules
A subcube definition is made up of one or more sets of members from different hierarchies
These sets can be included in a single SCOPE statement, or several nested SCOPE statements
They define the boundaries of the subcube on different hierarchies

15. Subcube definition rules

16. Subcube definition rules

17. Subcube definition rules

18. Subcube definition rules

19. Subcube definition rules
There are limitations on the shape of a subcube you can assign to
If you break them, you will get the infamous ‘arbitrary shaped set’ error
These are that a set of members can only contain:
One member from an attribute hierarchy
Several members from an attribute hierarchy, not including the All Member
Every member from an attribute hierarchy, including the All Member

20. Not allowed!

21. Subcube definition best practices
Best to ignore user hierarchies
Include sets of members from different hierarchies on the same dimension in the same SCOPE statement
Avoids problems with attribute overwrite
Specifically scope on one or more measures
Unless you really do want to scope on all measures
Even then can cause problems if you add new measures/measure groups in the future

22. Assignments to calculated members
Scoped assignments can be made to calculated members as well as real members
In fact, assignments to calculated measures are probably the most common use
Calculated measures don’t aggregate up…
You should not assign to a set containing both calculated members and real members
In most cases you can’t do this

23. Complex subcubes
Often we will want to scope on several attributes in a dimension at once
Eg all calendar year attributes, or all financial year attributes
To do this, you need to know your attribute relationships
The rule is:
For the lowest attribute, scope on all members in the hierarchy including the All Member
For the highest attribute, scope on all members in the hierarchy except the All Member

24. Controlling area of calculation
Calculated measures often need to do different things in different places
Some calculations should not be done at all in some places
Eg you may not want to see a 3 month moving average at the year level
Eg previous period growth for your first year with data
Some calculations may need different logic in different part of the cube
Eg tax calculations could be different every year
Eg calculation logic may change for business reasons, but you want to see the older algorithm used for historical data
Eg previous year growth could mean ‘previous financial year’ or ‘previous calendar year’ depending on which hierarchy you’re using

25. Assignments to real measures
When you assign to real (ie not calculated) measures, any values you assign will automatically aggregate up through the cube
This in turn may overwrite previous assignments that you have made
…and make it very difficult to know what’s going on…

26. Assignments and aggregation: example #1
120 30 40 10

27. Assignments and aggregation: example #1
120 30 40 10 20

28. Assignments and aggregation: example #1
120 30 40 50 10 20

29. Assignments and aggregation: example #1
130 30 40 50 10 20

30. Assignments and aggregation: example #1
120 30 40 10

31. Assignments and aggregation: example #1
500 30 40 10

32. Assignments and aggregation: example #1
500 30 40 10 20

33. Assignments and aggregation: example #1
500 or 130? 30 40 50 10 20

34. Assignments and aggregation: example #1
130 30 40 50 10 20

35. Special cases for aggregation
In some cases, a cell can be calculated in two different ways from previous assignments
When this happens, the assignment made last in the script will take precedence
This is the ‘last pass wins’ rule
Also, when there are unary operators, these will always take precedence
This is the ‘closest pass wins’ rule

36. The ‘Last Pass Wins’ rule
120 30 40 10

37. The ‘Last Pass Wins’ rule
120 30 40 10 20

38. The ‘Last Pass Wins’ rule
120 30 40 50 10 20

39. The ‘Last Pass Wins’ rule
130 30 40 50 10 20

40. The ‘Last Pass Wins’ rule
130 30 40 50 10 20

41. The ‘Last Pass Wins’ rule
140 or 290? 70 80 50 10 20 40

42. The ‘Last Pass Wins’ rule
140 70 80 50 10 20 40

43. The ‘Closest Pass Wins’ rule60 15 40 10 20 5 + + -

44. The ‘Closest Pass Wins’ rule60 15 50 40 10 20 5 + + -

45. The ‘Closest Pass Wins’ rule60 15 50 40 10 20 5 + + -

46. Uses for assignments to real measures
There are several practical uses for making assignments to real measures:
Replacing custom member formulas, so you can have all your code in the MDX Script
Always scope on members on the lowest level first then work upwards
Performing calculations at a low level of granularity and them aggregating the results
Eg currency conversion

47. Assignment ‘Passes’
Each new assignment creates a new ‘pass’ in the script
Think of the cube like an onion, with each new pass as a new layer in the skin on the outside
When you refer to an existing cell value in an assignment, you are actually referring to that cell value in the previous pass
Therefore avoiding infinite recursion

48. The FREEZE statement
When you assign a value to a cell, you create something like a pointer to that cell
So you don’t copy the new values assigned subcube
So if the values the assignment points to subsequently change, the values in your subcube will change
Using the FREEZE statement on a subcube stops this happening
Do not use FREEZE as a substitute for correct ordering of calculations
There is a performance overhead for using it!

49. The IF statement
Conditional assignments can be made using the IF statement: SCOPE(MEASURES.SALES); IF MEASURES.SALES>100 THEN THIS=0 END IF; END SCOPE;
Note that even when the condition evaluates to false and no assignment is made, a new pass is created

50. Assigning cell property values
The following cell property values can also be directly assigned to: Format_String, Non_Empty_Behavior, Language, Font_Size, Font_Flags, Font_Name, Fore_Color, Back_Color
Error values can be assigned to cells using the ERROR statement
Useful when:
A single measure holds values in different currencies which should be formatted differently depending on the currency
Debugging complex subcube definitions

51. Thanks!