先初始化一些数据，表名为 test ,字段及数据为:
SQL执行结果为：每个 uid 都只有 3 条记录。
SQL语句为：SELECT *FROM test mainWHERE (SELECT COUNT(1) FROM test sub WHERE main.uid = sub.uid AND main.gid > sub.gid ) < 3;
MYSQL 按某个字段分组，然后取每组前3条记录
SQL执行结果为：每个 uid 都只有 3 条记录。 SQL语句为：SELECT *FROM test mainWHERE (SELECT COUNT(1) FROM test sub WHERE main.uid = sub.uid AND main.gid > sub.gid ) < 3;