Why do we need Alias tables in OBIEE....?

Alias tables are an important part of designing a Physical layer because they enable you to reuse an existing table more than once. 

An alias table is a physical table that references a different physical table as its source (called the original table).

There are several versions of advantages of Alias tables .Below are some noted points.

1. Avoid circular joins.
2. Reuse the same object more than once.
3. Same table can act both as dimension and fact using aliases..

 lets see some examples for the same.

Example 1: 
Consider we have a two dates in a fact (Order_date and Ship_Date) and its has to be joined to Time dimension on both the keys.Since we cannot join both dates to a single key we create an Alias to Time dimension and join the Fact with two time dimensions one with Order_Date and the other with Sales_Order


Example 2:

Lets say we have a table Sales with following columns.

                        Order_Number,Item,Order_date,customer,Order_Qty

Since the same table is having dimensions and facts,we take an alias Sales_Fact & Sales_Dim and join on Order_Number.