Joins: An Introduction

Joining is a vital part of data analysis, it allows two related datasets to be combined into a single table that can be used as the basis for analysis. Joining is vital to understand when working with data and tableau provides a couple of options for joining data. But first Its important to understand what joining options are available and what they do.

Joining can of course be found in SQL databases where there a much wider variety of joins available than the three that we will look at here. Joining in a rudimentary way can also be seen in Excel's VLOOKUP function which acts in a way similar to a left join. The three join types that we will be looking at are those that Tableau supports.

An Inner join takes two datasets, we’ll call them the left and the right, and using one or more keys creates a resulting table which contains only the data relating to those keys that appear in both tables.

A Left Join (or Right Join) returns a table containing all the keys of the left (or Right) and any data from the other table that relates to those keys, or null if there is no data related to the key in the Right (or Left) table.

The below example highlights what a Left Join would result in.

Finally an outer join returns all the data from both tables

We hope that you found this quick introduction to joining data useful; in the next part in this series we'll be looking at how to join tables from the same data source in tableau.

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