conditional mysql if inner join - mysql

Question

is possible to do a conditional join if a field is non-null?, ie if a field is null do a join and not return a value, and if a field is not null then do a join and return a value
SELECT CASE WHEN i.id_servicio is null THEN p.nombre as proveedor, origen_incidencia.nombre as origen, relativo_a.nombre as relativo, i . * , u.nombre AS usuario ELSE p.nombre as proveedor, origen_incidencia.nombre as origen, relativo_a.nombre as relativo, i . * , u.nombre AS usuario,s.nombre
end
from incidencias i
INNER JOIN usuarios AS u ON i.id_usuarios =19 AND i.id_usuarios = u.id
case when i.id_servicio is not null then
INNER JOIN servicios s ON s.id = i.id_servicio
end
INNER JOIN relativo_a ON relativo_a.id = i.id_relativo_a
INNER JOIN origen_incidencia ON origen_incidencia.id = i.id_origen_incidencia
INNER JOIN proveedores p ON p.id = i.id_proveedor

Answer 1

Yes, it's called an outer join.
There are a couple of flavors of those. In this particular case you can use a left outer join, often just referred to as left join.
This query will return all incidencias and will just return NULL for the name if there is no service linked to the -erm- incident (?).
SELECT
i.id,
s.nombre
FROM incidencias i
LEFT JOIN servicios s ON s.id = i.id_servicio
There is also the right join, which does the same, but the other way around (rows in first table are optional). Quite often, using right join is considered bad practice, since it is more confusing to read, especially when you combine it with left joins in the same query. I don't think there are cases where you must use it, since you can always replace it with a left join by just reversing the tables.