diff --git a/backend/migrations/020_exercise_skills_unique.sql b/backend/migrations/020_exercise_skills_unique.sql
new file mode 100644
index 0000000..255d2a2
--- /dev/null
+++ b/backend/migrations/020_exercise_skills_unique.sql
@@ -0,0 +1,7 @@
+-- Migration 020: Add UNIQUE constraint to exercise_skills
+-- Fixes: "no unique or exclusion constraint matching the ON CONFLICT specification"
+-- Required for ON CONFLICT (exercise_id, skill_id) in import_wiki.py
+
+ALTER TABLE exercise_skills
+    ADD CONSTRAINT exercise_skills_exercise_skill_unique
+    UNIQUE (exercise_id, skill_id);