Joins in Tableau: Choose the correct join type

Key Takeaways

as usual the first thing we need to do when starting a new project is load our data our to file sources are audio book reviews and audio book sales let's connect them to tableau here's the audio book sales file in the same folder we also have the audio book reviews file we'll need both so I'll click on add and we'll load the other file too in order to be able to use the information of both tables at the same time we'll have to combine them somehow what we want to obtain is a unique table containing information about all transactions for all audio books and the respective reviews left by clients whenever they chose to do so the transaction ID column is the perfect link between the two files as it is available in both and could serve as a common field based on which we could link the two tables together so one of the main aspects we'll explore in this lesson is how to create a connection between two tables in tableau please remember that we will distinguish the terms data connection and table connection table connection is performed by functionalities like joining or blending it refers to combining data from two or more data sources data connection instead represents a single source connection between tableau and a data source such as an Excel workbook or an SQL database when we imported the second file tableau did some of the work for us in the background and created a connection between the two files the symbol you see here is an indication we've created an inner join technically speaking when creating a join tableau sends a query to the database the join is implemented on the relevant tables at the database level and the output of the operation is brought back to tableau where it is ready to be used for analysis and inner joy means that the two tables we've added have a field in common and we can combine them using that field let me hover my mouse over the symbol tableau indicates that it has created an inner join using the parameter audiobook name we don't want that we want to link the two tables according to the transaction ID parameter as our analysis will be focused on user sales and reviews so I'll click on the inner join symbol which will open the join menu first off let me substitute the field we use in order to create a connection between the two tables I would like to create a connection using the transaction ID field and here we are here's a preview of the table we created through an inner join and transaction ID has the primary key as shown in the diagram when we create an inner join between two tables we consider their intersection the situation where the primary key is found in both tables to give you an example if a person who has a given transaction ID purchased an audiobook but did not leave a review then they won't be included in this table an inner join would require users to have purchased an audiobook to have left a rating and both the date of purchase and the date when the rating was posted to be known basically it contains only the rows in which we have a transaction ID a date of purchase a date of review a review rating and information regarding which was the audiobook that was purchased and rated for example all the cases when a person bought an audiobook but did not rate it are not considered do you think this information is important do we want to be able to understand how reviews impacted sales and whether we can see a pattern in total purchases with respect to views of course we'll need this information and if we use an inner join we are not going to have it therefore we'll need a different type of join one that includes the information about purchases and people who did not rate an audiobook so looking at the tiny diagram we have here I am going to select a left join given that the audio book sales file is on the left side we want to have a table that contains information regarding audiobook purchases all of the audiobook purchases and the cases when people who bought these books provided a rating and a left join is more suitable than an inner join in this case okay great in our next lesson we'll make a check and see whether the data we've connected to tableau is correct this will do for now thanks for watching