获取多个表中某一字段的值？

Question

一个database里有很多表，每个表都有一个相同的字段（比如'user'），如何获取所有表中'user'的值？

Answer 1

select a.user, b.user, c.user from a, b, c

Answer 2

select table1.user from table1 union select table2.user from table1;

Answer 3

select a.user as usera, b.user as userb from...