Why Does My SELECT * Query Result in an ORA-00918: Ambiguous Column Definition Error?

Resolving Oracle ORA-00918: Ambiguous column definition error

It is frustrating to encounter the ORA-00918 error when using the SELECT * statement. This error occurs when a column with the same name appears multiple times in the result set and Oracle cannot determine which column to retrieve.

The following SQL statement:

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

There is a problem with the

SELECT * clause because there are multiple columns named ID in the subquery:

• COACHES.ID
• PEOPLE.ID
• USERS.ID

To resolve this ambiguity, the query must explicitly select named columns. It is recommended to use column aliases to improve clarity and avoid problems when adding or renaming columns in the future. For example:

SELECT
  COALESCE(COACHES.ID, PEOPLE.ID, USERS.ID) AS ID,
  people.*,
  users.*,
  coaches.*,
  organizations_users.*
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

This adjustment ensures there is only one instance of the ID column in the result set, resolving the ORA-00918 error and ensuring data integrity. COALESCE The function is used to handle the situation where NULL values ​​may exist and select the first non-NULL ID value. However, a better solution is to specify all required columns explicitly and avoid using the * wildcard character.

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