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

src/Query.php

@@ -647,7 +647,8 @@ public function derive($relation, Model $source): static
         $query = $relation->getTargetClass()::on($this->getDb());
         $resolver = $query->getResolver();
 
-        $reversed = $relation->reverse($resolver);
+        $reversed = $relation->reverse($resolver)
+            ->setJoinType('INNER');
 
         // This will fail if the name ("self") is occupied, but that's fine…
         $resolver->getRelations($query->getModel())->add($reversed);
@@ -724,6 +725,10 @@ public function createSubQuery(Model $target, string $targetPath, ?Model $from =
                 $oppositeRelation->setName($predecessor);
             }
 
+            // 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');
+
             $relations = new Relations();
             $relations->add($oppositeRelation);
             foreach ($subQueryResolver->getRelations($previousHop) as $detour) {

tests/Lib/Model/Chair.php

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

tests/QueryTest.php

@@ -6,9 +6,11 @@
 use ipl\Orm\NoopQuery;
 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
 {
@@ -652,4 +654,26 @@ public function testDeriveReturnsAValidButEmptyQueryIfNoLinkIsPossible(): void
         $this->assertEmpty(iterator_to_array($profile));
         $this->assertNull($profile->first());
     }
+
+    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']
+        );
+    }
 }

tests/RelationTest.php

@@ -307,11 +307,11 @@ public function testADeclaredInverseRelationCanBeReusedDuringReverse()
             <<<'SQL'
             SELECT relationship.id, relationship.coupler
             FROM relationship
-            LEFT JOIN loose relationship_self ON relationship_self.coupler = relationship.coupler
+            INNER JOIN loose relationship_self ON relationship_self.coupler = relationship.coupler
             WHERE (relationship_self.coupler = ?)
               AND ((relationship.id NOT IN ((SELECT sub_loose_relationship.id AS sub_loose_relationship_id
                  FROM loose sub_loose
-                 LEFT JOIN relationship sub_loose_relationship ON sub_loose_relationship.coupler = sub_loose.coupler
+                 INNER JOIN relationship sub_loose_relationship ON sub_loose_relationship.coupler = sub_loose.coupler
                  WHERE (sub_loose.id = ?) AND (sub_loose_relationship.id IS NOT NULL)
                  GROUP BY sub_loose_relationship.id
                  HAVING COUNT(DISTINCT sub_loose.id) >= ?)) OR relationship.id IS NULL))

tests/VisibilityFilterTest.php

@@ -367,7 +367,7 @@ public function testDeriveAppliesTheModelVisibilityFilterAndTheRelationFilter()
             'SELECT employee.id, employee.name, employee.active, employee.deleted,'
             . ' employee.role, employee.department_id, employee.office_id'
             . ' FROM employee'
-            . ' LEFT JOIN department employee_self'
+            . ' INNER JOIN department employee_self'
             . ' ON (employee_self.id = employee.department_id) AND (employee.active = ?)'
             . ' WHERE (employee.deleted = ?) AND (employee_self.id = ?)',
             $derived->assembleSelect(),