diff --git a/modoboa_installer/database.py b/modoboa_installer/database.py
index bb782a9..38fb1c7 100644
--- a/modoboa_installer/database.py
+++ b/modoboa_installer/database.py
@@ -10,8 +10,7 @@ from . import system
 from . import utils
 
 
-class Database(object):
-
+class Database:
     """Common database backend."""
 
     default_port: Optional[int] = None
@@ -157,7 +156,6 @@ class PostgreSQL(Database):
 
 
 class MySQL(Database):
-
     """MySQL backend."""
 
     default_port = 3306
@@ -178,7 +176,7 @@ class MySQL(Database):
         if name.startswith("debian"):
             if version.startswith("8"):
                 self.packages["deb"].append("libmysqlclient-dev")
-            elif version.startswith("11") or version.startswith("12"):
+            elif int(version[:2]) >= 11:
                 self.packages["deb"].append("libmariadb-dev")
             else:
                 self.packages["deb"].append("libmariadbclient-dev")
diff --git a/modoboa_installer/scripts/files/dovecot/2.4/conf.d/auth-sql-mysql.conf.ext.tpl b/modoboa_installer/scripts/files/dovecot/2.4/conf.d/auth-sql-mysql.conf.ext.tpl
index ffcc25a..9e6fef6 100644
--- a/modoboa_installer/scripts/files/dovecot/2.4/conf.d/auth-sql-mysql.conf.ext.tpl
+++ b/modoboa_installer/scripts/files/dovecot/2.4/conf.d/auth-sql-mysql.conf.ext.tpl
@@ -104,21 +104,21 @@ mysql %dbhost {
 #
 # Commonly used available substitutions (see https://doc.dovecot.org/latest/core/settings/variables.html
 # for full list):
-#   %{user} = entire user@domain
-#   %{user|username} = user part of user@domain
-#   %{user|domain} = domain part of user@domain
+#   %%{user} = entire user@domain
+#   %%{user|username} = user part of user@domain
+#   %%{user|domain} = domain part of user@domain
 # 
 # Note that these can be used only as input to SQL query. If the query outputs
 # any of these substitutions, they're not touched. Otherwise it would be
-# difficult to have eg. usernames containing '%' characters.
+# difficult to have eg. usernames containing '%%' characters.
 #
 # Example:
 #   query = SELECT userid AS user, pw AS password \
-#     FROM users WHERE userid = '%u' AND active = 'Y'
+#     FROM users WHERE userid = '%%u' AND active = 'Y'
 #
 #  query = \
 #    SELECT userid as username, domain, password \
-#    FROM users WHERE userid = '%{user|username}' AND domain = '%{user|domain}'
+#    FROM users WHERE userid = '%%{user|username}' AND domain = '%%{user|domain}'
 #}
 
 passdb sql {