IMCA - Introduction to RDBMS

A weak entity in a relational database is an entity that cannot be uniquely identified by its own attributes alone.

It relies on a relationship with another entity, known as the strong entity , to establish its identity.

This means a weak entity does not have its own primary key. Instead, it uses a partial key (also called a discriminator) along with the primary key of the strong entity to form its composite primary key . For example, consider an Invoice and its InvoiceLine. On its own, a line item like "Line 5" isn't unique — many invoices can have a "Line 5". So, to identify a specific line, you also need to know which invoice it belongs to. Therefore, InvoiceLine is a weak entity that depends on Invoice. In an Entity-Relationship diagram, weak entities are represented using double rectangles or dashed underlines. They cannot exist independently; if the corresponding strong entity is deleted, the weak entity typically gets deleted too.