What is the main purpose of Lookup transformation in data flow task?
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The lookup task is simply for looking up values from a table based on a provided lookup key(s). I provide an employee number and I can get back new fields into my data flow for name, departmnet, etc. This is the genric purpose it has. Now, the specific purpose of the Lookup Task is to be the key component in a surrogate key pipeline. When processing facts into a data warehouse, you need to replace the business/natural key with the surrogate key from the dimension table. The pipeline is a data flow task with lots of lookups to accomplish this. The lookup task also has the ability to direct lookup failures onto a seperate branch of the data flow to deal with late arriving dimensions / early arriving facts. In other words, if the lookup fails, you can substitute in some other value. In the case of late arriving dimensions, you can assign a surrogate key and put a place holder in the dimension table for the dimension that is late.