MySQL ERROR 1066 (42000): Not unique table/alias: 'x'
The same table or alias appears twice in a FROM/JOIN without distinct aliases.
Seen on:
SQL
Meaning
Self-joins and repeated joins need different aliases. ORMs joining the same relation twice can produce this.
Common causes
- Self-join without aliases
- Joining the same table twice
- ORM eager loads duplicating joins
⚡ Quick fix
- Give each occurrence its own alias (u1, u2)
- Check ORM eager-loading config if the SQL is generated
Detailed fix by platform
SQL
SELECT e.name, m.name AS manager FROM employees e JOIN employees m ON m.id = e.manager_id;
How to diagnose
- Alias — Which name repeats?
- Recent change — Did it start after a deploy, upgrade, or configuration change? Compare with the last working version.
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Last updated 2 Oct 2026