Refs #13312 -- Simplified handling of nulls ordering on MySQL.

MySQL & MariaDB support the standard IS NULL and IS NOT NULL so
the same workaround used for NULLS FIRST and NULLS LAST that is
used for SQLite < 3.30.0 can be used.

Thanks Simon Charette for the discussion.

django/db/backends/base/features.py

@@ -88,6 +88,9 @@ class BaseDatabaseFeatures:
     # Does the backend order NULL values as largest or smallest?
     nulls_order_largest = False
 
+    # Does the backend support NULLS FIRST and NULLS LAST in ORDER BY?
+    supports_order_by_nulls_modifier = True
+
     # The database's limit on the number of query parameters.
     max_query_params = None
 

django/db/backends/mysql/features.py

@@ -50,6 +50,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     db_functions_convert_bytes_to_str = True
     # Neither MySQL nor MariaDB support partial indexes.
     supports_partial_indexes = False
+    supports_order_by_nulls_modifier = False
 
     @cached_property
     def _mysql_storage_engine(self):

django/db/backends/sqlite3/features.py

@@ -44,3 +44,4 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_over_clause = Database.sqlite_version_info >= (3, 25, 0)
     supports_frame_range_fixed_distance = Database.sqlite_version_info >= (3, 28, 0)
     supports_aggregate_filter_clause = Database.sqlite_version_info >= (3, 30, 1)
+    supports_order_by_nulls_modifier = Database.sqlite_version_info >= (3, 30, 0)

django/db/models/expressions.py

@@ -1128,11 +1128,17 @@ class OrderBy(BaseExpression):
         return [self.expression]
 
     def as_sql(self, compiler, connection, template=None, **extra_context):
-        if not template:
+        template = template or self.template
+        if connection.features.supports_order_by_nulls_modifier:
             if self.nulls_last:
-                template = '%s NULLS LAST' % self.template
+                template = '%s NULLS LAST' % template
             elif self.nulls_first:
-                template = '%s NULLS FIRST' % self.template
+                template = '%s NULLS FIRST' % template
+        else:
+            if self.nulls_last:
+                template = '%%(expression)s IS NULL, %s' % template
+            elif self.nulls_first:
+                template = '%%(expression)s IS NOT NULL, %s' % template
         connection.ops.check_expression_support(self)
         expression_sql, params = compiler.compile(self.expression)
         placeholders = {
@@ -1144,23 +1150,6 @@ class OrderBy(BaseExpression):
         params *= template.count('%(expression)s')
         return (template % placeholders).rstrip(), params
 
-    def as_sqlite(self, compiler, connection):
-        template = None
-        if connection.Database.sqlite_version_info < (3, 30, 0):
-            if self.nulls_last:
-                template = '%(expression)s IS NULL, %(expression)s %(ordering)s'
-            elif self.nulls_first:
-                template = '%(expression)s IS NOT NULL, %(expression)s %(ordering)s'
-        return self.as_sql(compiler, connection, template=template)
-
-    def as_mysql(self, compiler, connection):
-        template = None
-        if self.nulls_last:
-            template = 'ISNULL(%(expression)s), %(expression)s %(ordering)s '
-        elif self.nulls_first:
-            template = 'IF(ISNULL(%(expression)s),0,1), %(expression)s %(ordering)s '
-        return self.as_sql(compiler, connection, template=template)
-
     def as_oracle(self, compiler, connection):
         # Oracle doesn't allow ORDER BY EXISTS() unless it's wrapped in
         # a CASE WHEN.