问题描述：
数据库进行了版本升级（5.5.31 -> 5.7.27），兼容旧数据时报错
Column count of mysql.user is wrong. Expected 45, found 42. Created with MySQL 50531
原因分析：
错误是由于曾经升级过数据库，升级完后没有使用mysql_upgrade升级数据结构造成的。
解决方案：
使用mysql_upgrade命令 : mysql_upgrade -u root -p
[seemmo@aimaster data]$ mysql_upgrade -u root -p
Enter password:
Checking if update is needed.
Checking server version.
Running queries to upgrade MySQL server.
Checking system database.
mysql.columns_priv OK
mysql.db OK
mysql.engine_cost OK
mysql.event OK
mysql.func OK
mysql.general_log OK
mysql.gtid_executed OK
mysql.help_category OK
mysql.help_keyword OK
mysql.help_relation OK
mysql.help_topic OK
mysql.innodb_index_stats OK
mysql.innodb_table_stats OK
mysql.ndb_binlog_index OK
mysql.plugin OK
mysql.proc OK
mysql.procs_priv OK
mysql.proxies_priv OK
mysql.server_cost OK
mysql.servers OK
mysql.slave_master_info OK
mysql.slave_relay_log_info OK
mysql.slave_worker_info OK
mysql.slow_log OK
mysql.tables_priv OK
mysql.time_zone OK
mysql.time_zone_leap_second OK
mysql.time_zone_name OK
mysql.time_zone_transition OK
mysql.time_zone_transition_type OK
mysql.user OK
Upgrading the sys schema.
Checking databases.
Upgrade process completed successfully.
Checking if update is needed.