A dimension contains two or more logical levels. The recommended sequence for creating logical levels is to create a grand total level and then create child levels, working down to the lowest level. The following are the parts of a dimension:
- Grand total level. A special level representing the grand total for a dimension. Each dimension can have just one Grand Total level. A grand total level does not contain dimensional attributes and does not have a level key. However, you can associate measures with a grand total level. The aggregation level for those measures will always be the grand total for the dimension.
- Level. All levels, except the Grand Total level, need to have at least one column. However, it is not necessary to explicitly associate all of the columns from a table with logical levels. Any column that you do not associate with a logical level will be automatically associated with the lowest level in the dimension that corresponds to that dimension table. All logical columns in the same dimension table have to be associated with the same dimension.
- Hierarchy. In each business model, in the logical levels, you need to establish the hierarchy (parent-child levels). One model might be set up so that weeks roll up into a year. Another model might be set up so that weeks do not roll up. For example, in a model where weeks roll up into a year, it is implied that each week has exactly one year associated with it. This might not be true for calendar weeks, where the same week could span two years. Some hierarchies might require multiple elements to roll up, as when the combination of month and year roll up into exactly one quarter. You define the hierarchical levels for your particular business so that results from analyses conform to your business needs and requirements.
- Level keys. Each logical level (except the topmost level defined as a Grand Total level) needs to have one or more attributes that compose a level key. The level key defines the unique elements in each logical level. The dimension table logical key has to be associated with the lowest level of a dimension and has to be the level key for that level. A logical level may have more than one level key. When that is the case, specify the key that is the primary key of that level. All dimension sources which have an aggregate content at a specified level need to contain the column that is the primary key of that level. Each logical level should have one level key that will be displayed when an Answers or Intelligence Dashboard user clicks to drill down. This may or may not be the primary key of the level. To set the level key to display, select the Use for drill down check box on the Level Key dialog box.Be careful using level keys such as Month whose domain includes values January, February, and so on—values that are not unique to a particular month, repeating every year. To define Month as a level key, you also need to include an attribute from a higher level, for example, Year. To add Year, click the Add button in this dialog and select the logical column from the dialog that is presented.