Single line with an arrow mark in Entity Relattion

问题:

What do single line with an arrow mark in ER diagram represent?

回答1:

As per Bachman Notation which you are using ,

"For every relation a rectangle has to be drawn and every coupling is illustrated by a line that connects the relations. On the edge of each line arrows indicate the cardinality. We have 1-to-n, 1-to-1 and n-to-m"

And "Cardinality use to represent the relationship between two entities."

For more on cardinality Notations

回答2:

E.R diagrams represent entities as boxes, and relationships as lines between the boxes. The ends of these lines are shaped to represent the cardinality of the relationship

an arrow from entity set to relationship set indicates a key constraint, i.e. injectivity: each entity of the entity set can participate in at most one relationship in the relationship set;

Have a look at these resources also for better understanding

example1

example2

回答3:

the simple answer is that it means your two entities who both are part of the same relationship, one instance of them must be part of the relationship but the one who the arrow points towards can exist only once or not at all. For example a president have a head but not all presidents use them for chess. So the relationship between chess games and president should be a arrow pointing from presidents to a relationship personPlayingChess single arrow towards Chessgames.