链表求和有关问题_Sql Server

链表求和问题
2个表
a：
id,user_name,regtime
b:
id,pay,user_name
现在表a的user_name和b是关联的
想查出表a的所有记录，再加上个字段
是在表b里的支付次数，我知道sum函数，但是忘了怎么关联了，哪位大侠指导一下！

------解决方案--------------------
from a left join b on a.user_name=b.user_name

------解决方案--------------------

SQL code

select     a.id,    a.user_name,    a.regtime,    sum(b.Pay)from a left join b on a.user_name=b.user_namegroup by a.id,a.user_name,a.regtime
------解决方案--------------------
SQL codeselect a.id,a.user_name,a.regtime,sum(b.pay)from aleft join b on a.user_name=b.user_namegroup by a.id,a.user_name,a.regtime
------解决方案--------------------
SQL codeselect a.id,a.user_name,a.regtime,sum(pay)from aleft join b on a.user_name=b.user_namegroup by a.id,a.user_name,a.regtime