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Each relationship between two tables is represented by a line connecting the parent table shape with the child table shape. Depending on the auto-layout options, the foreign key's name is optionally displayed next to the line.
The child side of the connection is pictured by a solid dot.
The parent side of the connection line leads directly into the parent shape when the relation is mandatory. That happens only if all foreign-key columns in the child table have the not null attribute.
A box shape is used to indicate the parent side of an optional connection.
Relationships based on a foreign key with the cascading delete option use a red line (as in the first illustration above).