When an additional table is added to your segment you must pick a unique identifier that lives on both tables to join.
When trying to understand joins, visualize two circles joined together. The join is where the circle intersects, which is where the unique identifier on both tables comes into play. Unique IDs are the recommended and most reliable join fields, but email and many other fields are often utilized. Use the most unique field possible that both tables have in common. In the example below Company ID on the Contacts table matches ID on the Company table.
To join the system generated Contacts table and the Companies table there is a Company ID on each table which can be used to join the tables together, assuming your have imported company ID data. Utilizing the Join will pull any contact on the Contacts table with a company ID if the ID also lives on the Company table. Utilizing the Left Outer Join will pull any contact according to the segment criteria whether or not they have a company ID.
The Events table houses activities related to email, form, social and web engagement. Every Contact that enters ROI is assigned a unique ROI ID found in the ID field of the Contacts table. The Events table in ROI is a system generated table that also includes ROI ID titled Contact ID.
The Events and Contacts table are joined on ROI ID as seen in the example below. One contact can have many or no activities that live on the Events table. For example, email@example.com has received 30 different ROI emails, submitted five ROI forms and regularly visits your website that includes the ROI capture script in the footer. That means a lot of activities have been tracked on the Events table for firstname.lastname@example.org. Because it is a one to many relationship (one contact, many activities) you will notice the count often dramatically increases when the Events table is joined to the Contacts table.
Additional join example:
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Contact email@example.com with additional questions.