UNION在mysql中被称为集合操作，操作类型分为两种：UNION DISTINCT 和  UNION ALL；注意：UNION和UNION DISTINCT是一样的功能。UNION功能为合并多个查询的结果并去重，UNION ALL的功能为合并多个查询的结果不去重。

集合操作时，两边的输入必须拥有相同的列数，如果数据类型不同的话，mysql会自动进行隐式转化 ；同时，结果列的名称由第一个查询的列的名称决定。下面看一下例子：

sql语句为：SELECT 'abc' as o,'haha' as p,4 as q FROM DUAL UNION SELECT 'abc' as a,'haha',4 FROM DUAL;结果是：

如果是：SELECT 'abc' as o,'haha' as p,4 as q FROM DUAL UNION ALL SELECT 'abc' as a,'haha',4 FROM DUAL;

如果两次查询的列个数不一致，如：SELECT 'abc' as o,'haha' as p,4 as q FROM DUAL UNION ALL SELECT 'abc' as a,'haha' FROM DUAL;