语句如下：

create_tablegroup_stmt:
    CREATE TABLEGROUP [IF NOT EXISTS] tablegroup_name;
alter_tablegroup_stmt:
    ALTER TABLEGROUP tablegroup_name ADD [TABLE] table_name_list;
drop_tablegroup_stmt:
    DROP TABLEGROUP [IF EXISTS] tablegroup_name;
table_name_list:
    table_name [, table_name ...]