How can we combine database and flat file data in Tableau desktop?

Connect to the first source, then in the data source page use Add next to Connections and connect to the second. Both then appear in the canvas and you can join them on a common field — a cross-database join.

Tableau handles the mechanics by bringing the data together in its own engine, so you can join a SQL Server table to an Excel sheet directly.

The alternative is blending, where the two remain separate data sources and are combined on a linking field at the level of the view. That is the better choice when the granularities differ, or when a join would duplicate rows.

In general, prefer the cross-database join when you need row-level access to both sides, and blend when you do not.