Merge dimension:

Merging of two common dimension from different table to 
retrieve some values is known as merging dimension.

For Example:

Table 1: contains column Empid and Emp name
Table 2:Contains column Empid and salary

you have Requirement that the client need the emp name and 
salary .

In this case we have to create two data provider one for 
retrieving the data from table 1 and other for table 2.
and by merging Empid in the two data providers you can get 
the value for emp name and salary