diff --git a/sql/upgrades/020.sql b/sql/upgrades/020.sql
new file mode 100644
index 00000000..d5c001d4
--- /dev/null
+++ b/sql/upgrades/020.sql
@@ -0,0 +1,61 @@
+-- SWH DB schema upgrade
+-- from_version: 19
+-- to_version: 20
+-- description: add table aliases to "where not exists" queries
+
+insert into dbversion(version, release, description)
+      values(20, now(), 'Work In Progress');
+
+CREATE OR REPLACE FUNCTION swh_directory_missing() RETURNS SETOF sha1_git
+    LANGUAGE plpgsql
+    AS $$
+begin
+    return query
+	select id from tmp_directory t
+	where not exists (
+	    select 1 from directory d
+	    where d.id = t.id);
+    return;
+end
+$$;
+
+CREATE OR REPLACE FUNCTION swh_release_missing() RETURNS SETOF sha1_git
+    LANGUAGE plpgsql
+    AS $$
+begin
+    return query
+        select id from tmp_release t
+	where not exists (
+	select 1 from release r
+	where r.id = t.id);
+    return;
+end
+$$;
+
+CREATE OR REPLACE FUNCTION swh_revision_missing() RETURNS SETOF sha1_git
+    LANGUAGE plpgsql
+    AS $$
+begin
+    return query
+        select id from tmp_revision t
+	where not exists (
+	    select 1 from revision r
+	    where r.id = t.id);
+    return;
+end
+$$;
+
+CREATE OR REPLACE FUNCTION swh_skipped_content_missing() RETURNS SETOF content_signature
+    LANGUAGE plpgsql
+    AS $$
+begin
+    return query
+	select sha1, sha1_git, sha256 from tmp_skipped_content t
+	where not exists
+	(select 1 from skipped_content s where
+	    s.sha1 is not distinct from t.sha1 and
+	    s.sha1_git is not distinct from t.sha1_git and
+	    s.sha256 is not distinct from t.sha256);
+    return;
+end
+$$;