3408
MariaDBERRORNotableForeign KeysHIGH confidence
Foreign key parent table has no index for the referenced column
Production Risk
Low — DDL fails; no constraint is created.
How to reproduce
trigger — this will error
trigger — this will error
ALTER TABLE child ADD FOREIGN KEY (col) REFERENCES parent(unindexed_col);
expected output
ERROR 3408 (HY000): Cannot add foreign key; referenced column has no index.
Fix
Add unique index to parent referenced column
Add unique index to parent referenced column
ALTER TABLE parent ADD UNIQUE INDEX idx_col (unindexed_col); ALTER TABLE child ADD FOREIGN KEY (col) REFERENCES parent(unindexed_col);
Why this works
A unique index on the parent column satisfies the InnoDB FK referential integrity requirement.
What not to do
✕
Sources
Official documentation ↗
MySQL 8.0 — 3408 ER_FK_NO_INDEX_PARENT
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