SOLID or DASHED/DOTTED

In Entity-Relationship Diagrams (ERDs)—such as those in MySQL Workbench, SQL Server Management Studio, or standard information engineering models—the choice between a solid line and a dashed (or dotted) line defines how tables relate to one another structurally.

1. Solid Line (Identifying Relationship)

A solid line indicates an identifying relationship.

• What it means: The child table cannot be uniquely identified without its parent table.
• Database impact: The Primary Key (PK) of the parent table migrates to become part of the Primary Key of the child table (forming a composite primary key, or inheriting it entirely).
• Example: Consider an Order table and an Order_Item table. An Order_Item cannot exist independently without knowing which Order it belongs to. Therefore, the order_id from the Order table joins the item_id to form the primary key of Order_Item.

2. Dashed or Dotted Line (Non-Identifying Relationship)

A dashed line indicates a non-identifying relationship.

• What it means: The child table has its own independent unique identifier (primary key) and can exist separately from the parent table.
• Database impact: The Primary Key of the parent table migrates down to the child table as a standard Foreign Key (FK), but it plays no part in the child table’s primary key.
• Example: Consider an Employee table and a Department table. An employee_id might reference which department_id they work in, but an employee has their own unique primary key (employee_id) and can even exist temporarily without a department assignment (if the foreign key allows NULL values).

Summary Comparison Table

Feature	Solid Line (Identifying)	Dashed Line (Non-Identifying)
Relationship Type	Strong / Identifying	Weak / Non-Identifying
Foreign Key Role	Becomes part of the Child's Primary Key	Stays strictly as a Foreign Key
Child Dependency	Existence-dependent (Weak entity)	Existence-independent
Common Use Case	Composite keys, associative lookup tables, weak entities	Standard master-detail/lookup relationships

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Are you trying to map out a specific database schema or troubleshooting a relationship style in a tool like MySQL Workbench?

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