sql三个表查询
三张记录表a1、a2、a3，每个表存储一个活动的参加人员记录，每个表中均有activeName字段表示参加人员的名称，该字段均为唯一性列，查询至少参加了2个两个的人名。 SQL
[解决办法]

select 
activeName 
from (
select 'a1' as TabName,activeName from a1
union all
select 'a2' as TabName,activeName from a2
union all
select 'a3' as TabName,activeName from a3
) as a
group by activeName 
having count(distinct TabName)>1

--如果 activeName在各表中是唯一的可以直接 COUNT(1)>=2
SELECT activeName,COUNT(1) FROM
(
  SELECT activeName FROM A1
  UNION ALL SELECT activeName FROM A2
  UNION ALL SELECT activeName FROM A3
)TB
GROUP BY activeName HAVING COUNT(1)>=2