EC
1066
MariaDBERRORCommonQueryHIGH confidence

Not unique table/alias in query

Production Risk

Low — query is rejected; no data is affected.

What this means

ER_NONUNIQ_TABLE (1066, SQLSTATE 42000) is raised when a query references the same table (or uses the same alias) more than once in the FROM clause without distinguishing aliases.

Why it happens
  1. 1The same table appears twice in a JOIN without distinct aliases
  2. 2A subquery or CTE uses the same alias as an outer table
How to reproduce
trigger — this will error
trigger — this will error
SELECT * FROM orders, orders;

expected output

ERROR 1066 (42000): Not unique table/alias: 'orders'

Fix

Assign unique aliases to each table reference

WHEN Self-join or repeated table reference is intentional.

Assign unique aliases to each table reference
SELECT a.id, b.id
FROM orders AS a
JOIN orders AS b ON a.customer_id = b.customer_id AND a.id <> b.id;

Why this works

Unique aliases disambiguate each table reference so MySQL can resolve column names correctly.

What not to do

✕ Remove one of the table references to fix 1066 without understanding the query

Self-joins are intentional in some queries; removing a reference changes query semantics.

Sources
Official documentation ↗

MySQL 8.0 — 1066 ER_NONUNIQ_TABLE

Content generated with AI assistance and reviewed for accuracy. Found an error? hello@errcodes.dev

← All MariaDB errors