My question is basically:
When to use which struct?

sqlc will generate per-query structs when they don't match any of the existing tables. Can you provide more information about your setup? What version of sqlc are you using? Can you post your schema, the problematic query, or the sqlc.json file?

I'm using version 1.5 with the dolphin parser.
Here's an example:

schema/persons.sql:

CREATE TABLE `persons` (
 `id` int NOT NULL AUTO_INCREMENT,
 `firstname` varchar(255) DEFAULT NULL,
 `lastname` varchar(255) DEFAULT NULL,
 `role` varchar(255) DEFAULT NULL,
 `shortname` varchar(255) DEFAULT NULL,
 `visitorpass_id` varchar(255) DEFAULT NULL,
 `funktion` varchar(255) DEFAULT NULL,
 `device` int DEFAULT NULL,
 `assignment_active` tinyint(1) DEFAULT '0',
 `deleted` datetime DEFAULT NULL,
 `tenant` int DEFAULT NULL,
 PRIMARY KEY (`id`),
 UNIQUE KEY `id` (`id`),
 UNIQUE KEY `id_2` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

query/person.sql:

/* name: ListPersons :many */
select
 id,
 firstname,
 lastname,
 shortname,
 visitorpass_id,
 funktion,
 device,
 assignment_active
from persons
where deleted is null
and role = ?
and tenant = ?;

sqlc.yaml:

version: "1"
packages:
 - name: "db"
 path: "db/"
 queries: "./sql/query/"
 schema: "./sql/schema/"
 engine: "mysql:beta"
 emit_json_tags: true
 emit_prepared_queries: true
 emit_interface: false
 emit_exact_table_names: false

After sqlc generate, the compiled function for ListPersons looks like this:

func (q *Queries) ListPersons(ctx context.Context, arg ListPersonsParams) ([]ListPersonsRow, error) {
 [...]
}

So, it is returning []ListPersonsRow.
In db/models.go, there is a Person struct, but none of the generated code is using it. Is this model only for me to use?

Hmm, so when looking into this in more detail, I realised:
If I select * or select all columns (and in the absolute same order as in the schema), ListPersons is returning []Person. In all other cases, it returns []ListPersonsRow. That's probably why (and doing so by design).

My main confusion is still coming from the fact that in case of passing on the data (e.g. via a JSON REST API), I still need to work with all the sql.NullXY datatypes (e.g. transform them to normal string / int64 etc.). At first I thought, this library (which I still like) helps to circumvent having to work with the sql.NullXY datatypes.