Why ERD Tools Misread 1:1 Relationships as 1:N — And How to Fix It

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A persistent flaw in many ERD tools causes one-to-one database relationships to be displayed incorrectly as one-to-many. The root cause is that a true 1:1 relationship in SQL is defined by a foreign key paired with a unique constraint, but most tools derive cardinality from the foreign key alone while ignoring the unique constraint. This bug was first reported against MySQL Workbench in 2009 and surfaced again with dbdiagram in May 2025, affecting both SQL importers and diagram exporters. The failure means a diagram and the underlying database can silently disagree on a fundamental structural fact, such as whether a user can have multiple profiles. Correctly reading cardinality requires tools to evaluate the full set of constraints on a column, not just the presence of a foreign key.

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