如何在 union all 中使用 order by

Posted 2023-05-09

【中文标题】如何在 union all 中使用 order by

【英文标题】：How to use order by in union all

【发布时间】：2020-03-20 10:27:57

【问题描述】：

我有 2 个表，我使用 union all 来合并这两个表。 我有 strCreatedOn 列作为时间戳。现在我无法在其中使用 order by 子句

select nVendorId,
        'bank'as change_column,   
        nRequestType as change_type,
        strCreatedOn as Timestamp,
        nItemState as status       
from vaamoz_admin.tblbankdetails
 where   nItemState = '1' AND nRequestType='4'

union all

select nVendorId,
        'kyc' as change_column, 
        nRequestType as change_type ,
        strCreatedOn as Timestamp,
          nItemState as status

 from vaamoz_admin.tblkycdetails
 where nItemState = '1' AND nRequestType='4'

【问题讨论】：

用您正在使用的数据库标记您的问题。

@GordonLinoff 是的。

【参考方案1】：

mysql 期望在每个查询周围加上括号，然后是 order by，如 in the documentation 所述：

要使用ORDER BY 或LIMIT 子句对整个UNION 结果进行排序或限制，请将各个SELECT 语句括起来，并将ORDER BY 或LIMIT 放在最后一个语句之后。

所以：

(
    select 
        nVendorId,
        'bank'as change_column,   
        nRequestType as change_type,
        strCreatedOn as Timestamp,
        nItemState as status       
    from vaamoz_admin.tblbankdetails
    where nItemState = 1 and nRequestType = 4
) union all (
    select 
        nVendorId,
        'kyc', 
        nRequestType,
        strCreatedOn,
        nItemState
    from vaamoz_admin.tblkycdetails
    where nItemState = 1 AND nRequestType = 4
 )
 order by Timestamp

注意事项：

不需要在第二个查询中重复列名（第一个查询中定义的名称会在另一个查询中产生）

nItemState 和 nRequestType 看起来像数字，所以应该这样处理（我删除了数字周围的单引号）

【讨论】：

MySQL 期望每个查询都带有括号 不。在这种情况下，单独查询的括号是多余的（但仍然可以使用它们）。它们必须仅在您想将排序（和限制）应用于单独查询而不是组合行集时使用。

@Akina：是的。问题是关于（外部）排序依据，需要括号。

没有。请阅读示例下方的句子。 不带括号的语句等同于刚才显示的带括号的语句。

我的评论字体中用斜体写的文字直接引用自参考手册，通过您的答案中提供的链接。您可以通过网页搜索来验证这一点。

【参考方案2】：

 select t.* from 
(
select nVendorId,
        'bank'as change_column,   
        nRequestType as change_type,
        strCreatedOn as Timestamp,
        nItemState as status       
from vaamoz_admin.tblbankdetails
 where   nItemState = '1' AND nRequestType='4'

union all

select nVendorId,
        'kyc' as change_column, 
        nRequestType as change_type ,
        strCreatedOn as Timestamp,
          nItemState as status

 from vaamoz_admin.tblkycdetails
 where nItemState = '1' AND nRequestType='4'
) as t order by t.Timestamp

【参考方案3】：

在大多数数据库中，您只需添加一个order by：

select nVendorId, 'bank'as change_column, nRequestType as change_type,
       strCreatedOn as Timestamp, nItemState as status       
from vaamoz_admin.tblbankdetails
where nItemState = '1' AND nRequestType='4'
union all
select nVendorId, 'kyc' as change_column, nRequestType as change_type ,
       strCreatedOn as Timestamp, nItemState as status
from vaamoz_admin.tblkycdetails
where nItemState = '1' AND nRequestType='4'
order by Timestamp;

在某些情况下，您需要一个子查询：

select t.*
from ((select nVendorId, 'bank'as change_column, nRequestType as change_type,
             strCreatedOn as Timestamp, nItemState as status       
       from vaamoz_admin.tblbankdetails
       where nItemState = '1' AND nRequestType='4'
      ) union all
      (select nVendorId, 'kyc' as change_column, nRequestType as change_type ,
           strCreatedOn as Timestamp, nItemState as status
       from vaamoz_admin.tblkycdetails
       where nItemState = '1' AND nRequestType='4'
      )
     ) t
order by Timestamp;

Here 是一个使用 MySQL 的简单示例。