获取多个表中某一字段的值？

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？

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

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

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