Relationship
A relationship: how two or more entities are related, such as a customer who places an order. In Chen notation it is a diamond with the relationship’s name inside, joined by lines to the entities it relates.
Easily confused with
How to read it
Relationships are the verbs of an ER diagram, where entities are the nouns: the diagram reads as a sentence, “employee works at factory”. The lines carry no arrows, and the layout implies no direction. Numbers beside the lines give the cardinality: 1:1, 1:N or M:N. A double line from an entity to the diamond means total participation: every record of that entity takes part. A diamond with a double outline is an identifying relationship, the one a Weak entity depends on.
The diamond is Peter Chen’s: his 1976 paper drew each relationship set as a “diamond-shaped box”.
Also searched as: er diagram diamond, relationship symbol.
Look-alikes
A diamond on a flowchart is a decision, where the flow branches on a yes or no. In a UML activity diagram a diamond is a decision or merge node. Neither has entities or attributes attached to it.
Differences by notation
Crow’s foot diagrams have no relationship diamond: a relationship is a line between two boxes, with the cardinality drawn at its ends, from One and only one to Zero or many. Because of that, a relationship among three entities, which Chen’s paper allowed, can’t be drawn as a single line in Barker’s crow’s foot notation.
Read next
- Next in ER diagram symbolsAttributeAn attribute: a property recorded for each entity, such as a customer’s name or an order’s date. In Chen notation as most database textbooks draw it, an attribute is an oval with its name inside, joined by a line to its entity.
- The same shape in Diamond gradingBlue white (diamond term)An old trade term for a diamond’s color, now limited by the FTC to stones that show no color other than blue or bluish.
More ER diagram symbols
The same shape elsewhere
Browse symbols drawn with diamonds or letters and numbers.
Sources
- The Entity-Relationship Model: Toward a Unified View of Data, Peter Pin-Shan Chen, ACM Transactions on Database Systems 1, no. 1 (March 1976): 9–36, Association for Computing Machinery (copy hosted by California State University, Bakersfield), Section 3.1, pp. 19–20
- A Practical Introduction to Databases: 2.2 Entity-relationship diagrams, Christopher Painter-Wakefield, 2022, Runestone Academy, 2.2.1.4 Relationships and 2.2.1.5 Cardinality ratios
- Data Modeling Using the Entity-Relationship (ER) Model (chapter 3 slides), Ramez Elmasri and Shamkant B. Navathe, Fundamentals of Database Systems, 2016, Pearson, course CS 448, Purdue University, Slides 3-28 and 3-42
- Entity Relationship Modeling in Database Management Systems, page 4, Richard Holowczak, 2013, updated 2022, Holowczak.com Tutorials, Oracle Designer (Crow’s Foot) or Barker-Ellis Notation
- Database System Concepts, 7th edition, chapter 6 slides: Database Design Using the E-R Model, Abraham Silberschatz, Henry F. Korth and S. Sudarshan, db-book.com, Slide 6.15
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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