可以在 unions 中将多个查询将结果合并成为一张结构相同数据表进行查询，可用于数据统计、分页展示等场景。

unions 数据结构为 Array<Object QueryDSL>

Gou Query DSL:

{
  "unions": [
    {
      "comment": "20只宠物猫",
      "select": ["id", "name"],
      "from": "cat",
      "limit": 20
    },
    {
      "comment": "20条宠物狗",
      "select": ["id", "name"],
      "from": "dog",
      "limit": 20
    },
    {
      "comment": "20条观赏鱼",
      "select": ["id", "name"],
      "from": "fish",
      "limit": 20
    }
  ],
  "limit": 10,
  "orders": "id"
}

解释为 SQL 语句 (MySQL):

 (SELECT `id`, `name` FROM `cat` LIMIT 20)
 UNION ALL (SELECT `id`, `name` FROM `dog` LIMIT 20)
 UNION ALL (SELECT `id`, `name` FROM `fish` LIMIT 20)
 ORDER BY `id`
 LIMIT 10