How to Resolve 'Column Reference is Ambiguous' Errors in SQL SELECT Statements?

Ambiguous column reference in SQL SELECT statement

In SQL, ambiguity occurs when multiple columns share the same name in different tables involved in a query. When these columns are selected, the columns referenced by the query are ambiguous, leading to errors such as "Ambiguous column reference."

Problem Statement

When trying to execute the following SELECT statement, you will encounter such a problem:

<code class="language-sql">SELECT (id,name) 
FROM v_groups vg 
INNER JOIN people2v_groups p2vg 
  ON vg.id = p2vg.v_group_id
WHERE p2vg.people_id = 0;</code>

This query attempts to retrieve the id and name columns from the v_groups table. However, the error message is returned: "The column reference id is ambiguous".

Cause of ambiguity

The ambiguity arises because both the v_groups and people2v_groups tables contain a column named id. Without specifying a table name or alias, the query cannot determine which id column should be retrieved.

Solution

To resolve ambiguities, explicitly specify the table name or alias before the column name in the SELECT part of the query. This makes it clear to the database which id column should be used.

Corrected query:

<code class="language-sql">SELECT (vg.id, name)
FROM v_groups vg 
INNER JOIN people2v_groups p2vg 
  ON vg.id = p2vg.v_group_id
WHERE p2vg.people_id = 0;</code>

By adding vg. before id, the query explicitly references the id column in the v_groups table, thus removing the ambiguity and allowing the query to execute successfully.

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