Oracle Error ORA-00918

Error code

ORA-00918

Product

Oracle Database

Versions

Oracle Database 19c, 21c, 23ai and 26ai (wording differs: see below)

Category

Objects, Names & Identifiers

Last reviewed

24 October 2026

What it means

ORA-00918 means a query joins two or more tables that both have a column with the same name, and the statement refers to that column without saying which table it comes from.

In a join, any column name that occurs in more than one of the tables must be prefixed with its table name or table alias. Oracle's reference uses the classic case: if EMP and DEPT are joined and both have DEPTNO, every reference must be EMP.DEPTNO or DEPT.DEPTNO (or an alias such as E.DEPTNO). Without the prefix Oracle cannot decide which column you mean and refuses to guess.

The rule applies to every clause of the query: the SELECT list, WHERE, GROUP BY, ORDER BY and join conditions. Current releases name the column and two of the tables in the message, which points straight at the fix; 19c and 21c print only "column ambiguously defined", so you have to find the unqualified column yourself. Shared column names such as ID, NAME, STATUS, CREATED_AT and foreign key columns are the usual suspects.

Common causes

• A join selects a column that exists in both tables (for example CUSTOMER_ID in ORDERS and CUSTOMERS) without a table alias.
• WHERE, GROUP BY or ORDER BY uses an unqualified column that is shared by joined tables, even though the SELECT list is qualified.
• A new column is added to one table with a name another joined table already uses, so a query that used to work starts failing.
• A query joins the same table twice (a self-join) and refers to a column without the alias of the copy it means.

Diagnose it

Read-only These queries only read the data dictionary (USER_, ALL_ and DBA_ views) and dynamic performance views (V$). They do not change data, privileges or settings, so they are safe to run first. DBA_ and V$ views need the matching privileges.

SELECT column_name, LISTAGG(table_name, ', ') WITHIN GROUP (ORDER BY table_name) AS in_tables
FROM all_tab_columns
WHERE owner = 'SHOP'
  AND table_name IN ('ORDERS', 'CUSTOMERS')
GROUP BY column_name
HAVING COUNT(*) > 1
ORDER BY column_name;

How to fix it

Each fix is labelled with what it changes. Try read-only checks and the least invasive fix first, and test changes outside production.

SELECT o.order_id, o.customer_id, c.name
FROM shop.orders o
JOIN shop.customers c ON c.customer_id = o.customer_id
WHERE o.status = 'OPEN'
ORDER BY o.order_id;

SELECT order_id, customer_id, name
FROM shop.orders
JOIN shop.customers USING (customer_id);

SELECT o.status AS order_status, c.status AS customer_status
FROM shop.orders o
JOIN shop.customers c ON c.customer_id = o.customer_id;

Examples

Selecting the join column without an alias

Not reproduced here: no Oracle Database is installed for this site. The result follows the current message format in Oracle's error reference, which names the column and two of the joined tables; the names are filled in from this example, and the exact form (table name or alias, quoting) is illustrative. Oracle Database 19c prints "ORA-00918: column ambiguously defined".

SELECT order_id, customer_id, name
FROM orders o
JOIN customers c ON c.customer_id = o.customer_id;

ORA-00918: CUSTOMER_ID: column ambiguously specified - appears in ORDERS and CUSTOMERS

An unqualified column in WHERE

Not reproduced here: no Oracle Database is installed for this site. The result is shown in the 19c form from Oracle's error reference; current releases add the column and table names. The SELECT list is qualified, but the WHERE clause is not, which is enough to raise the error.

-- both tables have a STATUS column
SELECT o.order_id, c.name
FROM orders o
JOIN customers c ON c.customer_id = o.customer_id
WHERE status = 'OPEN';

ORA-00918: column ambiguously defined

How to prevent it

Make it a habit to alias every table in a join and prefix every column, in every clause. Many teams enforce this in code review because it keeps queries working when someone adds a column with a common name such as STATUS or UPDATED_AT to one of the tables.

Name foreign key columns after the key they reference (customer_id in both tables) so joins read naturally, and use USING or qualified names for them consistently.

Common questions

Learn more on CodeWithSQL

• SQL joins guide
• Inner, left, right and full joins

Official documentation

• ORA-00918 (Oracle Database Error Messages)
• Joins (Oracle SQL Language Reference 19c)
• SELECT (Oracle SQL Language Reference 19c)

Last technically reviewed 24 October 2026; examples and diagnostics run on Oracle Database Error Messages reference (26ai, 21c and 19c); not run locally. Messages and explanations follow Oracle's Database Error Messages reference; no Oracle Database was available, so the examples were not run locally.