Comparison
SCD type 1vsSlowly changing dimension
SCD type 1
you overwrite the old value, and last year's report now shows the customer as having always been in Bristol.
Update the attribute in place and keep no history. It is the cheapest and it is right whenever the change is a correction rather than an event — a misspelled name, a fixed category. It is wrong whenever someone will re-run an old report and expect it to match what they printed, which is why it silently rewrites history and why nobody notices until an audit.
Full entry →Slowly changing dimension
a customer moved from Leeds to Bristol, and now you have to decide whether their orders from last year happened in Leeds or in Bristol.
The general problem of what to do when a descriptive attribute changes, given that facts referring to it have already been recorded. The numbered types are the standard answers, and choosing between them is a question for whoever reads the report rather than for the modeller. It is the classic warehouse conversation because both answers are defensible and only one of them is what the finance team assumed.
Full entry →