4115
MySQLERRORNotableHIGH confidence
Referenced foreign key column has no unique index
Production Risk
Low — DDL fails; no data is modified.
Why it happens
- 1The referenced columns in the parent table do not have a UNIQUE or PRIMARY KEY constraint.
- 2The FK was defined referencing a non-key column.
Fix
Add a unique index on the referenced parent columns
Add a unique index on the referenced parent columns
ALTER TABLE parent ADD UNIQUE INDEX idx_col (col);
Why this works
A unique index on the referenced column satisfies the FK reference requirement.
What not to do
✕
Sources
Official documentation ↗
MySQL 8.0 — 4115 ER_FK_NO_UNIQUE_INDEX_PARENT
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