报错如下：

Expression #2 of SELECT list is not in GROUP BY clause and contains 
nonaggregated column ‘a.id’ which is not functionally 
dependent on columns in GROUP BY clause; this is incompatible with 
sql_mode=only_full_group_by

解决方法一：
用sql查询

select @@global.sql_mode
//查询结果 ： ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION

去掉ONLY_FULL_GROUP_BY，重新设置值。
但重启mysql后，失效

set @@global.sql_mode 
='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION';

解决方法二：
修改mysql(my.cnf)配置文件,在[mysqld]下添加，重启mysql即可

sql_mode=STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION