Consider we have the following datamodel:

We want to retrieve the Person object linked with the Project Space object through Member relationship for which the Project Role is set to Contributor (Member relationship attribute). If we apply the “print bus” MQL command on the Project Space object and we select the Member relationship (print bus 'Project Space' myProject - select from[Member].to.name), MQL engine gives us 4 objects (muUser1, myUser2, myUser3 and myUser4).

Now, consider the following MQL command :

print bus 'Project Space' myProject - select from[Member|attribute[Project Role] == Contributor].to.name

The MQL engine gives us only myUser3.