4. How To: Intersections, Unions, and Differences
Posted by JT Burns on 13 September 2013 01:33 PM

Intersections, Unions, and Differences

Lists of students may compared to other lists of students. For example, two lists may be compared to find students that exist in both lists, this is called an intersection (^) between two student lists. One way of describing an intersection is the use of the word 'and' in the definition of search criteria. For example, one list might contain students that are athletes and another list contains students that are eligible for a certain program. To find students that are eligible and are athletes, you would intersect the two lists to find the results.

Unions (+) are used to combine the list of students from one list with the students in another list. One way of describing a union is the the use of the word 'or' in the definition of the criteria. For example if you wanted a list Business Majors or English Majors, you would union the Business Major list with the English Majors list. Another way defining the result is that the Business Majors list is added to the English Majors list.

Differences (-) are used to remove the students in a list from the students in another list. Generally, the word 'not' is used when defining the lists of students using a difference. For example you could find all students that are Business Major but do not have a 4.0 GPA by having two Lists, one of Business Majors and another of 4.0 students. The result would be defined as Business Majors minus the 4.0 students. Another way of describing the result would be to say Business Majors that are NOT 4.0 Students.


Use the List Management Dialog to Manage Lists
The Trac system has a method for quickly making a comparison between two lists. Choose List Management from the Lists menu under List Options. In the window that appears you may view all the lists you have access to. At the top are place holders for two lists and an operation to perform on the two lists (^, + or -). View the result of the operation by clicking the results button. To choose what the two lists are, click on the downward pointing triangle in the List 1 or List 2 column for the lists that you would like to choose. After choosing two lists and an operation, click Results.

The student listing will then display the students that represents the intersection, union, or difference between the two student lists.

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