SQLSTATE 42712Severity lowLab verified

Incident brief

Duplicate table alias

Each table or alias in a FROM clause must have a unique name; using the same alias twice raises this duplicate-alias error.

Reproduced on PostgreSQL 18.4Verified 2026-07-24 (isolated lab, PostgreSQL 18.4)Reviewed by Verified against PostgreSQL 18.4 in an isolated lab environment

In 10 seconds

What: Duplicate table alias
What triggers it: Write a FROM clause that references the same table twice with the same alias.
The fix: Give each FROM entry a distinct alias.
Proof: Reproduced on PostgreSQL 18.4 → A single query reproduces SQLSTATE 42712; the repeated alias is rejected before the join runs.

The fix

What to do right now

The immediate, application-level response to this error.

Give each FROM entry a distinct alias.
When self-joining a table, alias each instance differently (t1, t2).

Fix — SQL

-- Give each FROM entry a distinct alias.
SELECT * FROM ref_demo a, ref_demo b;

Diagnose

See it live on the server

Run these against the affected instance to confirm the diagnosis before you act.

Standard triage — not specific to this error

These are canonical PostgreSQL system-catalog queries, shown as SQL to run. No sample output is attached because this is general triage, not a captured lab transcript.

This SQLSTATE does not have an error-specific live snapshot yet. These are the canonical system-catalog queries you run against the affected server to see the problem in real time — standard triage, not a reproduced transcript.

What is running right now

Active backends, how long each has been running, and what it is waiting on.

SELECT pid,
       state,
       wait_event_type,
       wait_event,
       now() - query_start AS running_for,
       left(query, 80)     AS query
FROM   pg_stat_activity
WHERE  state <> 'idle'
  AND  pid <> pg_backend_pid()
ORDER  BY running_for DESC NULLS LAST;

Who is blocking whom

Turn raw blocking PIDs into the actual queries on both sides of the wait.

SELECT blocked.pid    AS blocked_pid,
       blocked.query  AS blocked_query,
       blocking.pid   AS blocking_pid,
       blocking.query AS blocking_query
FROM   pg_stat_activity AS blocked
JOIN   LATERAL unnest(pg_blocking_pids(blocked.pid)) AS b(pid) ON true
JOIN   pg_stat_activity AS blocking ON blocking.pid = b.pid
WHERE  cardinality(pg_blocking_pids(blocked.pid)) > 0;

Locks that are still waiting

Every lock a backend has requested but not yet been granted.

SELECT l.pid,
       l.locktype,
       l.mode,
       l.granted,
       COALESCE(c.relname, l.transactionid::text) AS object
FROM   pg_locks l
LEFT   JOIN pg_class c ON c.oid = l.relation
WHERE  NOT l.granted
ORDER  BY l.pid;

Reproduce & verify

A real, single-session PostgreSQL reproduction

A literal transcript of SQL run against a live PostgreSQL instance in an isolated lab — the commands below are exactly what was executed.

1Write a FROM clause that references the same table twice with the same alias.
2PostgreSQL builds the range table and finds two entries with the same name.
3The statement aborts with SQLSTATE 42712.

A self-join reused the same alias for both copies of a table, and the planner could not tell the two apart.

Schema setup

CREATE TABLE ref_demo(label text);
INSERT INTO ref_demo VALUES ('a');

Reusing a table alias

SELECT * FROM ref_demo a, ref_demo a;

The session continues normally

SELECT 'ok' AS session_after_error;

What PostgreSQL actually returned

Schema setup output

CREATE TABLE
INSERT 0 1

Reusing a table alias output

ERROR:  table name "a" specified more than once

The session continues normally output

 session_after_error 
---------------------
 ok
(1 row)

Related & next steps

Follow the thread

Everything this error touches — jump straight to the sibling error, term, runbook, or parameter.

Verification

PG 18.4

Last verified: 2026-07-24 (isolated lab, PostgreSQL 18.4)
Reviewed by: Verified against PostgreSQL 18.4 in an isolated lab environment
Audit status: reviewed