## `union`
此方法用于指定UNION查詢
## 示例:
~~~
db.name('user').field('name').union('select name from info').select()
~~~
~~~
db.name('user')\
.field('name')\
.union('select name from info')\
.union('select name from work')\
.select()
~~~
~~~
db.name('user')\
.field('name')\
.union(['select name from info', 'select name from info'])\
.select()
~~~
>[danger] UNION 內部的 SELECT 語句必須擁有相同數量的列
> 列也必須擁有相似的數據類型
> 每條 SELECT 語句中的列的順序必須相同。
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