mariadb-左外联接

Question

recently have migrated a server, and I have found this "error", I had mysql as a DB, and what I wanted (I'm not an expert on SQL), was to join 2 related tables by 1:N, as an example,

Table 1: Badges_Person
Table 2: Badges

Badges is a table with the badges, and Badges_Person contains a relation like (id_badge, id_person), easy, uh? Well this SQL query always seemed to work fine:

SELECT id, nombre, descripcion, insignias.time, obtained
FROM insignias LEFT OUTER JOIN 
     (SELECT *, '1' as obtained
      FROM insignias_user
      WHERE insignias_user.username = 'Octal'
    ) as insignias_user_seleccionado
    ON insignias.id = insignias_user_seleccionado.id_insignia;

The output of this query was the list of badges with a 'obtained' column (0 or 1) which says if the user 'Octal' has that badge or not.

So..., now, I have mariadb as DB, and it returns a different output, where all the rows are being marked with 'obtained' = 1.

I came here because as far as I have tried I have discarded all the silly posible errors.

Answer 1

I cannot speak to why the query is not working. That would seem to be a data issue -- all the rows match.

But, there is a better way to write the query:

SELECT i.id, i.nombre, i.descripcion, i.time, ius.obtained
FROM insignias i LEFT OUTER JOIN 
     insignias_user iu
    ON i.id = ius.id_insignia AND ius.username = 'Octal';

This is much more efficient because the intermediate table does not need to be materialized and the database can make use of appropriate indexes on insgnias_user .

Also note: I changed the column references to qualified column names. The table alias may not be correct.

Answer 2

SELECT i.id, i.nombre, i.descripcion, i.time, IF(ius.id_insignia IS NULL, 0, 1)
FROM insignias i LEFT OUTER JOIN insignias_user ius
ON i.id = ius.id_insignia AND ius.username = 'Octal';

Ok, it works again, thank you.