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I have created a model in the Visual Data Model of MySQL Workbench, and on one single relationship, this icon shows up:

Circle relationship icon

What does it mean? The Relation is 1:1, and this icon shows only on this particular relationship, none other. I already checked and the relationship is exactly the same as all the other.

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up vote 5 down vote accepted

The icon shows that that side of the relationship is not mandatory, meaning the field can be NULL. You can change that if you click on the relationship line, go to Foreign Key tab and select the Mandatory check box.

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