Question

我试图从不同的表中获取文件夹和文件名，并使用EACH查询语句的结果，按其update_date的降序排序，然后使用UNION进行连接。

如果我分别运行这两个查询，它确实起作用，并且结果将根据日期的降序从数据库中检索。但是当我将UNION与它们一起使用时，它并没有给出正确的答案。

获取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。它将基于我从未想要的update_date ORDER BY DESC排序所有数据。

我想向用户显示文档列表，例如Google展示在Google云端硬盘中，始终显示“始终文件夹”，然后显示文件。他们每个人都按update_date DESC

排序

Answer 1

您可以尝试以下操作，其中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

在子查询中按使用顺序

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，始终将文件夹放在第一位，然后将文件放在第二位，因此将其分为两种类型以保持正确的顺序

如何将UNION与两个具有ORDER BY的语句一起使用

2 个答案: