How to Resolve Oracle's ORA-00918: Ambiguous Column Definition Error in SELECT * Queries?

*Oracle ORA-00918: Ambiguous Column Definition – A SELECT Query Problem**

Running a SELECT * query against multiple joined tables in Oracle can lead to the dreaded ORA-00918 error: "ambiguous column definition." This happens when your query includes columns with identical names from different tables. Oracle can't determine which column you intend to retrieve.

Here's an example query that often triggers this error:

<code class="language-sql">SELECT *
FROM
  (SELECT DISTINCT(coaches.id),
    people.*,
    users.*,
    coaches.*
  FROM "COACHES"
  INNER JOIN people ON people.id = coaches.person_id
  INNER JOIN users ON coaches.person_id = users.person_id
  LEFT OUTER JOIN organizations_users ON organizations_users.user_id = users.id
) WHERE rownum <= 25</code>

The Root of the Problem

The SELECT * attempts to retrieve all columns, but since id exists in multiple tables (coaches, people, users), Oracle is left uncertain which id column to select.

The Solution: Precise Column Specification

To fix ORA-00918, avoid SELECT *. Instead, explicitly list the columns you need. There are two primary ways to achieve this:

1. Direct Column Selection

Specify each column using its fully qualified name (table.column):

<code class="language-sql">SELECT coaches.id,
       people.name,
       users.username,
       coaches.team
FROM
  (SELECT DISTINCT(coaches.id),
    people.*,
    users.*,
    coaches.*
  FROM "COACHES"
  INNER JOIN people ON people.id = coaches.person_id
  INNER JOIN users ON coaches.person_id = users.person_id
  LEFT OUTER JOIN organizations_users ON organizations_users.user_id = users.id
) WHERE rownum <= 25</code>

2. Utilizing Column Aliases

Assign unique aliases to columns with the same name using the AS keyword:

<code class="language-sql">SELECT coaches.id AS coaches_id,
       people.name AS person_name,
       users.username AS user_username,
       coaches.team AS coaches_team
FROM
  (SELECT DISTINCT(coaches.id),
    people.*,
    users.*,
    coaches.*
  FROM "COACHES"
  INNER JOIN people ON people.id = coaches.person_id
  INNER JOIN users ON coaches.person_id = users.person_id
  LEFT OUTER JOIN organizations_users ON organizations_users.user_id = users.id
) WHERE rownum <= 25</code>

By employing either method, you provide Oracle with unambiguous instructions, resolving the ORA-00918 error and ensuring your query executes successfully. Remember to replace the example column names with your actual column names.

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