Mysql union联合查询

1.UNION 用于合并两个或多个 SELECT 语句的结果集，并消去表中任何重复行。
2.UNION 内部的 SELECT 语句必须拥有相同数量的列，列也必须拥有相似的数据类型,每条 SELECT 语句中的列的顺序必须相同.
3.UNION ALL允许有重复的行

SELECT column_name FROM table1 

UNION 

SELECT column_name FROM table2 

UNION ALL不消除重复行

SELECT column_name FROM table1

UNION ALL

SELECT column_name FROM table2

重复

(select * from talbe order by id) UNION ALL (select * from talbe order by id);

不重复

(select * from talbe order by id) UNION (select * from talbe order by id);