Query: Always use INNER join to establish the outer query link

src/Query.php

@@ -657,6 +657,10 @@ public function createSubQuery(Model $target, string $targetPath, ?Model $from =
         foreach ($this->getResolver()->resolveRelations($targetPath, $from) as $relationPath => $relation) {
             $predecessor = array_slice(explode('.', $relationPath), -2, 1)[0];
             foreach ($relation->reverse($subQueryResolver) as $oppositeRelation) {
+                // Always override the join type, as a sub-query that is unable
+                // to establish a link to the outer query is useless anyway.
+                $oppositeRelation->setJoinType('INNER');
+
                 if (
                     $relation->getReverseName() === null
                     && $predecessor !== $oppositeRelation->getName()

tests/Lib/Model/Chair.php

@@ -28,6 +28,7 @@ public function getColumns()
 
     public function createRelations(Relations $relations)
     {
-        $relations->belongsTo('employee', Employee::class);
+        $relations->belongsTo('employee', Employee::class)
+            ->setJoinType('LEFT');
     }
 }

tests/QueryTest.php

@@ -5,8 +5,11 @@
 use ipl\Orm\Exception\InvalidRelationException;
 use ipl\Orm\Query;
 use ipl\Orm\ResolvedExpression;
+use ipl\Sql\Connection;
 use ipl\Sql\Expression;
 use ipl\Sql\Test\TestCase;
+use ipl\Stdlib\Filter;
+use ipl\Tests\Orm\Lib\Model\Employee;
 
 class QueryTest extends TestCase
 {
@@ -607,4 +610,26 @@ public function testWithoutColumnsDoesNotWorkWithExpressions()
             $query->assembleSelect()
         );
     }
+
+    public function testSubQueriesOverrideTheJoinType(): void
+    {
+        $query = (new Query())
+            ->setDb($this->createStub(Connection::class))
+            ->setModel(new Employee())
+            ->columns('name')
+            ->filter(Filter::equal('chair.vendor', 'Icinga'));
+
+        $this->assertSql(
+            <<<'SQL'
+SELECT employee.name FROM employee
+WHERE (employee.deleted = ?) AND (employee.id IN ((SELECT sub_chair_employee.id AS sub_chair_employee_id
+    FROM chair sub_chair
+    INNER JOIN employee sub_chair_employee
+        ON (sub_chair_employee.id = sub_chair.employee_id) AND (sub_chair_employee.deleted = ?)
+    WHERE sub_chair.vendor = ?)))
+SQL,
+            $query->assembleSelect(),
+            ['n', 'n', 'Icinga']
+        );
+    }
 }