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数据库系统概念第7章，数据库设计和E-R模型
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SQL7.md

7.5 实体联系图

7.5.6 弱实体集

  • 没有足够属性以形成主码的实体集称作弱实体集(weak entity set)
  • 有主码的实体集称作强实体集(strong entity set)
  • 弱实体集必须与另一个称作**标识(identifying)或属主实体集(owner entity set)**的实体集关联才能有意义。
  • 每个弱实体集必须和一个标识实体集关联，也就是说弱实体集**存在依赖(existence dependent)**于标识实体集。
  • 我们称标识实体集**拥有(own)**它所标识的弱实体集。
  • 将弱实体集与其标识实体集相联的联系称为标识性联系(identifying relationship)。
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