Core Concept Mastery • DBMS Module
Let's look at a Bank Transaction.
Strong Entity: "Bank Account". It has an Account Number (PK). It can exist independently.
Weak Entity: "Transaction" (Deposit/Withdrawal). It doesn't have its own key. It has a Transaction ID, but if you don't know which account it belongs to, it's useless.
If you close your Account (Delete Strong Entity), your transaction history (Weak Entity) should also be deleted. Otherwise, you have 'ghost transactions'. This dependency is the Double Diamond relationship.
Imagine a "Transaction" record. It makes no sense without the "Bank Account" it belongs to. If the Account is deleted, the Transaction loses its meaning.
An "Employee" is a Strong Entity. Their "Dependent" (family member for insurance) is a Weak Entity—they are only in the system because of the Employee.
"The relationship between a strong and a weak entity is denoted by?"
Double Diamond (Identifying Relationship).