MySQL多行查询 并自定义值返回

select id,intShow,@a:='a' from a_data where `intShow` = 2
union all
select id,intShow,@a:='b' from b_data where `intShow` = 2
union all
select id,intShow,@a:='c' from c_data where `intShow` = 2
order by id desc

SELECT id,intShow,idSchool,'life' AS block,expireDate,date FROM `life` UNION ALL 
SELECT id,intShow,idSchool,'like' AS block,expireDate,date FROM `like` UNION ALL 
SELECT id,intShow,idSchool,'info' AS block,expireDate,date FROM `info` UNION ALL 
SELECT id,intShow,idSchool,'my' AS block,expireDate,date FROM `my` ORDER BY id DESC