ER Diagram Builder

An Entity-Relationship (ER) diagram is a visual model of a database schema. It shows entities (tables), their attributes (columns), and the relationships between them. ER diagrams are used during database design to clarify structure before writing SQL, communicate schema to teammates, and document existing databases.

A primary key (PK) uniquely identifies each row in a table — no two rows can share the same PK value, and it cannot be NULL. Common choices are auto-incrementing integers (id) or UUIDs. A foreign key (FK) is a column in one table that references the primary key of another table, establishing a link between the two. For example, a Post table might have a user_id FK pointing to the User table's id PK.

Cardinality describes how many instances of one entity relate to instances of another. One-to-one (1:1) means each row in table A corresponds to exactly one row in table B. One-to-many (1:N) means one row in A can relate to many rows in B — the most common type (e.g., one User has many Posts). Many-to-many (M:N) means rows in A can relate to multiple rows in B and vice versa, which requires a junction/pivot table.

Use one-to-many when ownership is clear and exclusive — one User owns many Posts, one Order contains many OrderItems. Use many-to-many when both sides can have multiple associations — a Student can enroll in many Courses and a Course can have many Students. Many-to-many relationships require a join table (e.g., enrollments) with foreign keys to both sides, and often additional attributes like enrollment date.

Crow's foot notation uses symbols at each end of a relationship line. A single vertical line (|) means 'exactly one'. A crow's foot (three lines splaying out) means 'many'. A circle means 'zero'. Combined symbols read as: single line + single line = one and only one; crow's foot + single line = one or many; crow's foot + circle = zero or many. Read the symbol at the far end of the line as the maximum, and the symbol closer to the entity as the minimum.