Definition of weak entity set: There are not enough attributes to form the entity set of the master code.
Strong entity set: entity set with master code
The weak entity set depends on the strong entity set and exists. The connection between them
Note: This contact has no descriptive attributes.
Considering the two entity sets,
we find that the attributes are redundant, and the loan number is the main code of the loan , as well as the external code and main code of the repayment.
In order to eliminate redundancy, we can only eliminate the loan number attribute of repayment .
But there will be a problem, who is the repayment? ?
To solve this problem, we set up a special link set repayment , it can provide to the repayment entity set an additional property, namely the loan number. Loan repayment here is called an iconic connection .
After becoming a weak entity set, the number is called a discriminator, which is used to distinguish different repayment entities in the repayment set .
It is with the loan numbers constitute a set repayment of the main code together.
Of course, if the weak entity set only participates in identifying connections and has few attributes, the weak entity set can also be designed as a multi-valued composite attribute.
