A brief analysis of the difference between drop user and delete from Mysql.user _mysql

Drop user deletes all of the user's information, and delete only clears the users table, and other information, such as the DB table, exists.
If you create a user with the least privilege after delete, then he reuses the previous permissions.
Grant all on test.* to ' test ' @ '% ' identified by ' test ';
Delete from user where user =test;
Look at DB table
Mysql> select * from DB;
+------+---------+------+-------------+-------------+-------------+-------------+-------------+-----------+---- --------+-----------------+------------+------------+-----------------------+------------------+--------------- ---+----------------+---------------------+--------------------+--------------+------------+--------------+
| Host | Db | User | Select_priv | Insert_priv | Update_priv | Delete_priv | Create_priv | Drop_priv | Grant_priv | References_priv | Index_priv | Alter_priv | Create_tmp_table_priv | Lock_tables_priv | Create_view_priv | Show_view_priv | Create_routine_priv | Alter_routine_priv | Execute_priv | Event_priv | Trigger_priv |
+------+---------+------+-------------+-------------+-------------+-------------+-------------+-----------+---- --------+-----------------+------------+------------+-----------------------+------------------+--------------- ---+----------------+---------------------+--------------------+--------------+------------+--------------+
| %    | test    |      | y           | y           | y           | y           | y           | y         | n          | y               | y          | y          | y                      | y                | y                | y              | y                   | n                  | n            | y          | y            |
| %    | test\_% |      | y           | y           | y           | y           | y           | y         | n          | y               | y          | y          | y                      | y                | Y                | y              | y                   | n                  | n            | y          | y            |
| %    | test    | Test | y           | y           | y           | y           | y           | y         | n          | y               | y          | y          | y                      | y                | y               | y              | y                   | y                  | y            | y          | y            |
+------+---------+------+-------------+-------------+-------------+-------------+-------------+-----------+---- --------+-----------------+------------+------------+-----------------------+------------------+--------------- ---+----------------+---------------------+--------------------+--------------+------------+--------------+
3 Rows in Set (0.00 sec)
Permission for test still exists
Create a user with only select test
Grant Select on test.* to ' test ' @ '% ' identified by ' test ';
Mysql> select * from DB;
+------+---------+------+-------------+-------------+-------------+-------------+-------------+-----------+---- --------+-----------------+------------+------------+-----------------------+------------------+--------------- ---+----------------+---------------------+--------------------+--------------+------------+--------------+
| Host | Db | User | Select_priv | Insert_priv | Update_priv | Delete_priv | Create_priv | Drop_priv | Grant_priv | References_priv | Index_priv | Alter_priv | Create_tmp_table_priv | Lock_tables_priv | Create_view_priv | Show_view_priv | Create_routine_priv | Alter_routine_priv | Execute_priv | Event_priv | Trigger_priv |
+------+---------+------+-------------+-------------+-------------+-------------+-------------+-----------+---- --------+-----------------+------------+------------+-----------------------+------------------+--------------- ---+----------------+---------------------+--------------------+--------------+------------+--------------+
| %    | test    |      | y           | y           | y           | y           | y           | y         | n          | y               | y          | y          | y                      | y                | y                | y              | y                   | n                  | n            | y          | y            |
| %    | test\_% |      | y           | y           | y           | y           | y           | y         | n          | y               | y          | y          | y                      | y                | Y                | y              | y                   | n                  | n            | y          | y            |
| %    | test    | Test | y           | y           | y           | y           | y           | y         | n          | y               | y          | y          | y                      | y                | y               | y              | y                   | y                  | y            | y          | y            |
+------+---------+------+-------------+-------------+-------------+-------------+-------------+-----------+---- --------+-----------------+------------+------------+-----------------------+------------------+--------------- ---+----------------+---------------------+--------------------+--------------+------------+--------------+
3 Rows in Set (0.00 sec)
permissions are the same as before.