如何将 UNION 与两个具有 ORDER BY 的语句一起使用

Question

我正在尝试从不同的表中获取文件夹和文件名，以及每个查询语句的结果，按其update_date 的降序排序，然后使用UNION 加入它。

如果我分别运行这两个查询，它确实有效，并且将根据日期的降序从数据库中检索结果。但是当我和他们一起使用UNION 时，它并没有给我正确的答案。

获得它的正确方法是什么？

(
    SELECT create_date, folder_name AS actual_name, folder_id, folder_displayed_name AS displayed_name, update_date, delete_flag, NULL AS img_url, NULL AS protect_flag, NULL AS file_type, list_order
    FROM aaaaa_estate.docs_folder
    WHERE delete_flag =0
    AND inside_of_folder =  ''
    ORDER BY update_date DESC

    )
    UNION (

    SELECT create_date, file_name AS actual_name, id, displayed_name, update_date, delete_flag, img_url, protect_flag, file_type, list_order
    FROM aaaaa_estate.docs_contract
    WHERE delete_flag =0
    AND inside_of =  ''
    ORDER BY update_date DESC
)

如果我在最后使用 Order By 子句。它根据他们的 update_date ORDER BY DESC 对所有数据进行排序，这是我永远不想要的。

我想向用户显示文档列表，例如 Google Drive 中的 Google 显示，Always Folder first and files second 。他们每个人都按update_date DESC排序

Answer 1

您可以尝试以下 ORDER BY 子句取决于两列：

SELECT * FROM
(
    SELECT 1 qry_nr, create_date, folder_name AS actual_name, folder_id, folder_displayed_name AS displayed_name, update_date, delete_flag, NULL AS img_url, NULL AS protect_flag, NULL AS file_type, list_order
    FROM aaaaa_estate.docs_folder
    WHERE delete_flag =0
    AND inside_of_folder =  ''


    UNION 

    SELECT 2 qry_nr, create_date, file_name AS actual_name, id, displayed_name, update_date, delete_flag, img_url, protect_flag, file_type, list_order
    FROM aaaaa_estate.docs_contract
    WHERE delete_flag =0
    AND inside_of =  ''
 -- ORDER BY update_date DESC
) q
ORDER BY q.actual_name, q.qry_nr;

Answer 2

在子查询之外使用 order by

select t.* from 
(
    SELECT 1 as type, create_date, folder_name AS actual_name, folder_id, folder_displayed_name AS displayed_name, update_date, delete_flag, NULL AS img_url, NULL AS protect_flag, NULL AS file_type, list_order
    FROM aaaaa_estate.docs_folder
    WHERE delete_flag =0
    AND inside_of_folder =  ''   

    UNION     
    SELECT 2 ,create_date, file_name AS actual_name, id, displayed_name, update_date, delete_flag, img_url, protect_flag, file_type, list_order
    FROM aaaaa_estate.docs_contract
    WHERE delete_flag =0
    AND inside_of =  ''

) t order by type asc, update_date desc

正如您提到的，您始终首先使用文件夹，然后将文件划分为两种类型以保持正确的顺序