diff --git a/wagtail/wagtailimages/south_migrations/0004_auto__chg_field_rendition_focal_point_key.py b/wagtail/wagtailimages/south_migrations/0004_auto__chg_field_rendition_focal_point_key.py
index e1c17d342d..25202438eb 100644
--- a/wagtail/wagtailimages/south_migrations/0004_auto__chg_field_rendition_focal_point_key.py
+++ b/wagtail/wagtailimages/south_migrations/0004_auto__chg_field_rendition_focal_point_key.py
@@ -13,11 +13,22 @@ class Migration(SchemaMigration):
 
     def forwards(self, orm):
         # remove duplicate renditions
-        db.execute("""
-            DELETE FROM wagtailimages_rendition WHERE image_id || '-' || filter_id IN (
-                SELECT image_id || '-' || filter_id FROM wagtailimages_rendition WHERE focal_point_key IS NULL GROUP BY image_id, filter_id HAVING COUNT(*) > 1
-            ) AND focal_point_key IS NULL
-        """)
+        if db._get_connection().vendor == 'mysql':
+            db.execute("""
+                DELETE FROM wagtailimages_rendition WHERE CONCAT(image_id, '-', filter_id) IN (
+                    SELECT CONCAT(image_id, '-', filter_id) FROM (SELECT * FROM wagtailimages_rendition) as x
+                    WHERE focal_point_key IS NULL
+                    GROUP BY image_id, filter_id HAVING COUNT(*) > 1
+                ) AND focal_point_key IS NULL
+            """)
+        else:
+            db.execute("""
+                DELETE FROM wagtailimages_rendition WHERE image_id || '-' || filter_id IN (
+                    SELECT image_id || '-' || filter_id FROM wagtailimages_rendition 
+                    WHERE focal_point_key IS NULL
+                    GROUP BY image_id, filter_id HAVING COUNT(*) > 1
+                ) AND focal_point_key IS NULL
+            """)
 
         # Changing field 'Rendition.focal_point_key'
         db.alter_column('wagtailimages_rendition', 'focal_point_key', self.gf('django.db.models.fields.CharField')(max_length=255, default=''))