mysql> select sn,id,info from s100 group by id;+-----+------+------+| sn  | id   | info |+-----+------+------+| 227 |    1 | aa   || 228 |    2 | bb   |+-----+------+------+2 rows in set (0.02 sec)mysql> select sn,count(id),info from s100 group by id;+-----+-----------+------+| sn  | count(id) | info |+-----+-----------+------+| 227 |     32768 | aa   || 228 |         1 | bb   |+-----+-----------+------+2 rows in set (0.02 sec)mysql> mysql> mysql> show index from s100;+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+| s100  |          0 | PRIMARY  |            1 | sn          | A         |       33366 |     NULL | NULL   |      | BTREE      |         |               |+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+1 row in set (0.00 sec)mysql>  show variables like '%tx_isolation%';+---------------+----------------+| Variable_name | Value          |+---------------+----------------+| tx_isolation  | READ-COMMITTED |+---------------+----------------+1 row in set (0.00 sec)mysql> show variables like '%commit%';+--------------------------------+-------+| Variable_name                  | Value |+--------------------------------+-------+| autocommit                     | OFF   || binlog_order_commits           | ON    || innodb_api_bk_commit_interval  | 5     || innodb_commit_concurrency      | 0     || innodb_flush_log_at_trx_commit | 1     |+--------------------------------+-------+5 rows in set (0.00 sec)Session 1:mysql> select * from s100 where id=2 for update;+-----+------+------+| sn  | id   | info |+-----+------+------+| 228 |    2 | bb   |+-----+------+------+1 row in set (0.02 sec)Session 2:Database changedmysql> insert into s100(id,info) values(9999,'fcadfad');Query OK, 1 row affected (0.00 sec)RC模式下,是行锁 不会上升到表级锁