union命令作用：连接两个以上的 SELECT 语句的结果组合到一个表中
select id, url, auth_status as authStatus, enterprise_name as enterpriseName, establish_time as establishTime, ‘node‘ as type, null as messageId, null as sender, created_time from node where node_role != ‘SELF‘ union select id , null as url, null as authStatus, null as enterpriseName, null as establishTime, ‘message‘ as type, id as messageId, sender,created_time from message where `message_type`=‘INVITE‘ and handled=0 order by created_time desc
注意事项：
1. 两表字段长度需要一致
2. order by 排序的字段，需要在查询字段中包含
3. union 合并时默认去重，若不需要去重，可以使用union all
select sum(a.b) as num from ( select count() as b from table_1 union all select count() as b from table_2 ) as a（注意这里要取个别名）
原文：https://www.cnblogs.com/yishilin/p/12325429.html