ervice-now allows you to easily create relationships between tables without having to know a whole lot about the underlying table structure. If you’ve ever worked with a reference field then you’ve worked with one of these relationships. A reference field is simply a one-to-many relationship between 2 tables in ServiceNow. The ‘one’ end of the relationship is displayed as a reference field on the form. The ‘many’ end of the relationship is displayed on the referenced table as a related list.
In some cases, it may also be necessary to create a many-to-many relationship in your instance. Because ServiceNow ships with most of these relationships already defined, it’s rare that you’ll have to create one. If you do have to create a many-to-many relationship, here’s how you could do it.

For this example, we’ll assume that we need a many-to-many relationship between the Incident and the Change request tables. One Incident can be related to many changes (maybe several changes are required as a result of an incident) and one Change request can be related to many Incidents (implementation of a change results in many incidents).

1) Navigate to the m2m table
There are two tables in your ServiceNow instance that contain many-to-many relationship information. The first one (sys_collection) contains the out-of-box relationships and should be used only for reference purposes. The second one (sys_m2m) is where you’ll want to create your own many-to-many definitions. It’s probably a good idea to create your own modules to link to these tables so that you can easily access them if you’re going to be dealing with m2m tables very often. In most cases these tables won’t be used very much so it may be easier just to navigate to them directly in the application filter in your left navigation or by navigating directly by url. The easiest way is just to type ‘sys_m2m.list’ in your left navigation filter.

2) Create a new Many-to-Many relationship tableClick the ‘New’ button on the ‘sys_m2m’ table list and create a new record. There are really only 2 fields that you need to fill in since the rest gets automatically calculated for you. All you need to specify are the ‘From table’ and the ‘To table’ values. For this example we’ll specify ‘Incident’ as our ‘from table’ and ‘Change request’ as our ‘to table’. These values could be interchanged with no impact on the end result. You do want to pay attention to the values in the other fields though since they indicate what your table will be named along with how your related lists will be labeled. Although these are all calculated automatically, you can change them if needed.

3) Add the related list to your Change request table

Navigate to your Change request form, right-click the form header, and personalize the related lists for the form. Select the ‘Incidents’ list to add to your form. All One-to-Many lists have a ‘->’ separating the relationship. Many-to-Many lists (and custom formatters) have no separator.

4) Add the related list to your Incident tableFollow the same procedure for your Incident table. This list should show up as ‘Change requests’, but may show up with the name of your many-to-many table as shown below. If your related list doesn’t show up correctly, submit an incident to ServiceNow support.

5) Relate a record to test it outYour related lists will show up at the bottom of each form. Once you have added the related lists to both sides of the relationship, it is easy to relate records to each other using the ‘New’ and ‘Edit’ buttons on the related list.