All relationships in a relational database are handled through the use of foreign keys (FK). For a binary relationship, the FK in the child table points to a record in the parent table. For a unary relationship, the FK points to a parent record in the same table.
For example, we could represnt all people in a single table named People. Some people might be parents; some might be children. Some people might be grandparents and others might be great-great-great-grandchildren. The beauty of this single table represnetation is that any number of generations can be handled.
PID could be used as a unique primary key for the people in the table. Some people have mothers (in the table); some do not. MomID could be used as a pointer to the PID of each person's mother.
In order to draw this unary mother-child relationship in Visio, just connect both ends of a relationship to the People table. Unfortunately, Visio cannot determine which field should be the FK. You and I know it should be MomID, but Visio cannot read our minds.
We must manually define the mother-child relationship ourselves. First, highlight the relationship, then select PID and MomID.
Finally, click the Associate button.
That's all there is to it.
The mother-child unary relationship is 1:N. (Moms can have lots of kids.) What if you want to represent a 1:1 relationship like marriage. (Mom cannot have lots of husbands... at the same time.)
This is easy to change. Just highlight the relationship, then change the cardinality from "zero or more" to "zero or one."
