Question

how can i add underline for primary key in ER diagram? Thank you :)

Answer 1

"Underlined" is one of the properties for selected nodes/labels that can be enabled in the properties table in yEd's lower right corner. Aside from that, that is also a underlined template in the "Entity Relationship" palette section.

Comments

The "underline" property affects the whole text in the label. We need to underline only ONE LINE in the label to highlight it is the primery key for the entity

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In this case, use HTML formatting. Please see section Properties of Graph Elements in the yEd Manual for additional information.