实体关系图中带有箭头标记的单线
ER图中带箭头的单线代表什么？
What do single line with an arrow mark in ER diagram represent?
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ER图中带箭头的单线代表什么？
What do single line with an arrow mark in ER diagram represent?
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简单的答案是，这意味着您的两个实体都是同一关系的一部分，它们的一个实例必须是该关系的一部分，但箭头指向的实体只能存在一次或根本不存在。例如，总统有一个头脑，但并非所有总统都用它来下棋。因此，国际象棋游戏和总统之间的关系应该是一个从总统指向关系人下棋的箭头，指向国际象棋游戏的单箭头。
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.
它代表最多有 1 个参与关系，因此如果两个实体都有一个来自该关系的箭头，则为 1 对 1 关系，而当双方都有一条线时，则为 M对M的关系。
It represents at most 1 participating in the relation, so if both entities having an arrow from the relationship it will be 1 to 1 relationship and when there is a single line in both sides it will be M to M relationship.
根据您正在使用的 巴赫曼表示法，
“对于每个关系，都必须绘制一个矩形，并且每个耦合由连接关系的线表示。每条线的边缘箭头表示基数，我们有 1 对 n、1 对 1 和 n 对 m”
。
“基数用于表示两个实体之间的关系。”
有关基数表示法的更多信息
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
ER 图将实体表示为方框，将关系表示为方框之间的线。这些线的末端的形状表示关系的基数
从实体集到关系集的箭头表示关键约束，即单射性：实体集的每个实体最多可以参与关系集中的一个关系;
查看这些资源也可以更好地理解
用户Ed Gibbs<编辑 /a>.截至 2024 年 6 月，下面的两个链接均已损坏。
schemabank.com/help/notation-entity-relationship-diagram危险：此链接的域不再属于原始作者。smartdraw.com/resources/tutorials/cardinality-notations/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
Edited by user Ed Gibbs. Both links below are broken as of June 2024.
schemabank.com/help/notation-entity-relationship-diagramDANGER: this link's domain no longer belongs to the original author.smartdraw.com/resources/tutorials/cardinality-notations/