Explanation
When using a Blend, the primary data source appears with a BLUE tick-mark and the secondary data source appears with a ORANGE tick-mark.
See below:

Explanation
You can union your data to combine two or more tables by appending values (rows) from one table to another.
To union your data in Tableau data source, the tables must come from the same connection.
For example, suppose you have the following customer purchase information stored in three tables, separated by month. The table names are "May2016," "June2016," and "July2016."

A union of these tables creates the following single table that contains all rows from all tables.

Follow along the steps below:
Drag Assortment and Year ID to the column shelf, and Item Number ID to the row shelf. Next, drag Sales to the Text label to create a cross-tab as below:

Measure Names is a dimension. Latitude, Longitude, and Number of records are all measures.

Explanation
The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns.
Consider 2 tables "Orders" and "Customers".
If there are records in the "Orders" table that do not have matches in "Customers", these orders will not be shown!