SQL根据特定值的行数打印值

GROUP BY而且CASE应该够了

select
    t1.Account_Number,
    case when count(*) > 1 then 'Y' else 'N' end Is_multiple
from table1 t1
inner join table2 t2
on t1.meter_id = t2.meter_id
group by t1.Account_Number;

编辑:

根据修改后的架构,这应该工作:

select
    t1.Account_Number,
    case when count(*) > 1 then 'Y' else 'N' end Is_multiple
from table1 t1
inner join table2 t2
on t1.agr_id = t2.agr_id
inner join table3 t3
on t2.meter_id = t3.meter_id
group by t1.Account_Number;