SQL server 数据库中，如何将分组后元素用逗号隔开并作为一条记录？

创建表及数据 create table t1(mid int,uid varchar(1))insert into t1 values (1,'a')insert into t1 values (1,'b')insert into t1 values (1,'b')insert into t1 values (1,'c')insert into t1 values (1,'d')insert into t1 values (2