Transformation Rules (Cont.)

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Transformation Rule 3. N-N Relationships
When two entities E and F take part in an N-N relationship R:

• The many-to-many relationship R is mapped to a table T.


• The table T contains columns for all attributes in the primary identifiers of E and F.


• The set of columns forms the primary key for T.


• T also contains columns for all attributes attached to the relationship.

Below shows the many-to-many relationship, work, where an employee must work on at least one project, and a project could have no one working on it (idle).

The table work includes two columns from the primary keys of the two entities employee and project, and one column from its attribute percent.
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employee eid staddress city state zipcode 197 7 Beacon St Boston MA 02122 221 19 Brighton St Boston MA 02103 303 153 Mass Ave Cambridge MA 02123 ... ... ... ... ... project prid project_name due_date p11 Phoenix 3/31/22 p13 Excelssir 9/31/21 p21 White Mouse 6/30/21 ... ... ...		work eid prid percent 197 p11 50 197 p13 25 197 p21 25 221 p21 100 303 p13 40 303 p21 60 ... ... ...