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Every dimension member must be a unique value. Depending on your data, you can create a dimension that uses either natural keys or surrogate keys from the relational sources for its members. If you have any doubt that the values are unique across all levels, then keep the default selection of surrogate keys.
Source keys are read from the relational sources without modification. To use the same exact keys as the source data, the values must be unique across levels. Because each level may be mapped to a different relational column, this uniqueness may not be enforced in the source data. For example, a dimension table might have a Day column with values of 1
to 366
and a Week column with values of 1
to 52
. Unless you take steps to assure uniqueness, the values from the Week column overwrite the first 52 Day values.
Surrogate keys ensure uniqueness by adding a level prefix to the members while loading them into the analytic workspace. For the previous example, surrogate keys create two dimension members named DAY_1
and WEEK_1
, instead of a single member named 1
. A dimension that has surrogate keys must have at least one level-based hierarchy.