A join combines two tables into one, row by row, wherever a field matches. In Tableau, joins live in the physical layer: on the Data Source page, double-click the main table, then drag another table next to it. The Venn icon between them sets the type:
- Inner keeps only rows that match in both tables.
- Left keeps every row of the left table and fills the right table's fields with null where nothing matches.
- Right and Full Outer do the same for the right table, or for both.
The trap is the level of detail. Orders has one row per order line; Region Managers has one row per region. Joined on Region, each region's target is copied onto every order line, so SUM of the target adds it hundreds of times. Use an aggregation like AVG or MIN for such fields, or relate the tables instead (next lesson).
A union is different: it stacks tables with the same columns, like this year's file under last year's.