MySQL UNION 操作符用于连接两个以上的 SELECT 语句的结果组合到一个结果集合中
union 会删除重复数据
union all 不会删除重复数据
select * from
(
select *,'a' as kind from tablea where name is not null
union all
select *, 'b' as kind from tableb where name = 'b-data'
) a
union 会删除重复数据
union all 不会删除重复数据
select * from
(
select *,'a' as kind from tablea where name is not null
union all
select *, 'b' as kind from tableb where name = 'b-data'
) a