The entity set transaction has attributes transaction-number, date and amount. Different transactions on different accounts could share the same number. These are not sufficient to form a primary key (uniquely identify a transaction). Thus transaction is a weak entity set.
For a weak entity set to be meaningful, it must be part of a one-to-many relationship set. This relationship set should have no descriptive attributes.
The idea of strong and weak entity sets is related to the existence dependencies seen earlier.
Member of a strong entity set is a dominant entity.
Member of a weak entity set is a subordinate entity.
A weak entity set does not have a primary key, but we need a means of distinguishing among the entities.
The discriminator of a weak entity set is a set of attributes that allows this distinction to be made.
The primary key of a weak entity set is formed by taking the primary key of the strong entity set on which its existence depends (see Mapping Constraints) plus its discriminator.
transaction is a weak entity. It is existence-dependent on account. The primary key of account is account-number.
transaction-number distinguishes transaction entities within the same account (and is thus the discriminator). So the primary key for transaction would be (account-number, transaction-number). Just Remember: The primary key of a weak entity is found by taking the primary key of the strong entity on which it is existence-dependent, plus the discriminator of the weak entity set.