"Duplicate column name" error that shouldn't exist - mysql

Question

I'm trying to migrate a database from sqlite to mariaDB ( which I never worked with ) and I can't seem to get past this problem!
When I try to run this, I always get
"#1060 - Duplicate column name 'id_document'"
Here's the query:
SELECT
a.id_document AS id_document,
id_user_associatedDoc,
id_user_creator,
stage,
title,
goldStandardAnnotationManual,
count(content) AS answer_count
FROM
(SELECT *
FROM Document join DocumentProject
ON Document.id_document = DocumentProject.id_document
WHERE DocumentProject.id_project = 2) a
LEFT JOIN
(SELECT *
FROM Annotation
WHERE Annotation.id_project = 2) b
ON a.id_document = b.id_document
GROUP BY a.id_document;
I don't understand why I should be getting this error! Can you help?

Answer 1

This first subquery syntax returns two id_document column:
(SELECT *
FROM Document
join DocumentProject
ON Document.id_document = DocumentProject.id_document
WHERE DocumentProject.id_project = 2) a
A quick way to fix this:
SELECT
a.id_doc /*Change this column as following*/ AS id_document,
id_user_associatedDoc,
id_user_creator,
stage,
title,
goldStandardAnnotationManual,
count(content) AS answer_count
FROM
(SELECT *, Document.id_document as "id_doc" /*define the first id_document column as a different name*/
FROM Document
join DocumentProject
ON Document.id_document = DocumentProject.id_document
WHERE DocumentProject.id_project = 2) a
LEFT JOIN
(SELECT *
FROM Annotation
WHERE Annotation.id_project = 2) b
ON a.id_document = b.id_document
GROUP BY a.id_document;
You can also pre-define which column you want to return inside the subquery as well. Example:
(SELECT Document.id_document, Document.column_2, Document.column_3
FROM Document
join DocumentProject
ON Document.id_document = DocumentProject.id_document
WHERE DocumentProject.id_project = 2) a
That way, your outer query should select the a.id_document according to what you have pre-define inside the subquery and no need to change anything from the outer query.