---------------链式操作之union
UNION操作用于合并两个或多个 SELECT 语句的结果集。
使用示例：
Db::field('name')
->table('think_user_0')
->union('SELECT name FROM think_user_1')
->union('SELECT name FROM think_user_2')
->SELECT();
->table('think_user_0')
->union('SELECT name FROM think_user_1')
->union('SELECT name FROM think_user_2')
->SELECT();
数组用法：
Db::field('name')
->table('think_user_0')
->union(['field'=>'name','table'=>'think_user_1'])
->union(['field'=>'name','table'=>'think_user_2'])
->SELECT();
->table('think_user_0')
->union(['field'=>'name','table'=>'think_user_1'])
->union(['field'=>'name','table'=>'think_user_2'])
->SELECT();
或者
Db::field('name')
->table('think_user_0')
->union(['SELECT name FROM think_user_1','SELECT name FROM think_user_2'])
->SELECT();
->table('think_user_0')
->union(['SELECT name FROM think_user_1','SELECT name FROM think_user_2'])
->SELECT();
支持UNION ALL 操作，例如：
Db::field('name')
->table('think_user_0')
->union('SELECT name FROM think_user_1',true)
->union('SELECT name FROM think_user_2',true)
->SELECT();
->table('think_user_0')
->union('SELECT name FROM think_user_1',true)
->union('SELECT name FROM think_user_2',true)
->SELECT();
或者
Db::field('name')
->table('think_user_0')
->union(['SELECT name FROM think_user_1','SELECT name FROM think_user_2'],true)
->SELECT();
->table('think_user_0')
->union(['SELECT name FROM think_user_1','SELECT name FROM think_user_2'],true)
->SELECT();
每个union方法相当于一个独立的SELECT语句。
注意：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。同时，每条 SELECT 语句中的列的顺序必须相同。