Fix duration-only expressions for SQLite and MySQL

Allow arithmetic with DurationField on SQLite/MySQL by converting
timedelta expressions to microseconds for proper backend handling.
Addresses errors with duration-only annotation queries.

django/db/backends/mysql/operations.py

@@ -1,6 +1,7 @@
 import uuid
 
 from django.conf import settings
+from django.db import DatabaseError
 from django.db.backends.base.operations import BaseDatabaseOperations
 from django.utils import timezone
 from django.utils.duration import duration_microseconds
@@ -288,6 +289,24 @@ class DatabaseOperations(BaseDatabaseOperations):
             return 'FLOOR(%(lhs)s / POW(2, %(rhs)s))' % {'lhs': lhs, 'rhs': rhs}
         return super().combine_expression(connector, sub_expressions)
 
+    def combine_duration_expression(self, connector, sub_expressions):
+        # MySQL doesn't support direct arithmetic on INTERVAL expressions.
+        # For duration-only operations, we need to extract the numeric values
+        # and perform integer arithmetic, then format the result.
+        if connector not in ['+', '-']:
+            raise DatabaseError('Invalid connector for timedelta: %s.' % connector)
+        # Remove INTERVAL ... MICROSECOND wrapping if present and do numeric arithmetic
+        unwrapped = []
+        for expr in sub_expressions:
+            if expr.startswith('INTERVAL ') and expr.endswith(' MICROSECOND'):
+                # Extract the numeric part
+                unwrapped.append(expr[9:-12])  # Remove 'INTERVAL ' and ' MICROSECOND'
+            else:
+                unwrapped.append(expr)
+        # Perform the arithmetic and return as microseconds (BIGINT)
+        conn = ' %s ' % connector
+        return conn.join(unwrapped)
+
     def get_db_converters(self, expression):
         converters = super().get_db_converters(expression)
         internal_type = expression.output_field.get_internal_type()

django/db/backends/sqlite3/base.py

@@ -561,6 +561,10 @@ def _sqlite_format_dtdelta(conn, lhs, rhs):
         return None
     # typecast_timestamp returns a date or a datetime without timezone.
     # It will be formatted as "%Y-%m-%d" or "%Y-%m-%d %H:%M:%S[.%f]"
+    # For timedelta, return the value as microseconds to match the expected
+    # format for DurationField in databases without native duration support.
+    if isinstance(out, datetime.timedelta):
+        return duration_microseconds(out)
     return str(out)