在 Where 子句中存在的 SQL 查询答案

【问题标题】：SQL Query with Exists in Where Clause在 Where 子句中存在的 SQL 查询
【发布时间】：2021-09-21 20:38:33
【问题描述】：

我已经建立了一个查询，如果根据我的数据库中的某些字段被更改，在更改日志表中发现更改，我会提取记录。如果不进行更改，则不存在任何记录。我已经建立了我的查询，它可以很好地给我想要的东西，但问题是，当我将其缩小到一个日期范围时，如果更改日志中没有要检查的记录，我什么也得不到。有没有办法在 where 语句中执行某种“如果存在”来检查日期，如果记录不存在，仍然允许返回结果，因为我在 select 语句中编码了结果文本？这是我的代码：

Declare @StartDate as date
Declare @EndDate as date

Set @StartDate = '9/20/2021'
Set @EndDate = '10/1/2021'


Select DISTINCT n.ID, n.FULL_NAME, n.Company, n.Status, n.Email, 
    Case 
        when Coalesce(CLDP.CurrentValue, 'No Update Found') <> 'No Update Found' then CLDP.CurrentValue
        When Coalesce(CLDP.CurrentValue, 'No Update Found') = 'No Update Found' and b.PROFESSION <> '' then b.PROFESSION
        when Coalesce(CLDP.CurrentValue, 'No Update Found') = 'No Update Found' and b.PROFESSION = '' then 'No Update Found'
    end as ProfessionUpdate,

    Case 
        when Coalesce(CLDE.CurrentValue, 'No Update Found') <> 'No Update Found' then CLDE.CurrentValue
        When Coalesce(CLDE.CurrentValue, 'No Update Found') = 'No Update Found' and b.ETHNICITY <> '' then b.ETHNICITY
        when Coalesce(CLDE.CurrentValue, 'No Update Found') = 'No Update Found' and b.ETHNICITY = '' then 'No Update Found'
    end as EthnicityUpdate,

    Case 
        when Coalesce(CLDG.CurrentValue, 'No Update Found') <> 'No Update Found' then CLDG.CurrentValue
        When Coalesce(CLDG.CurrentValue, 'No Update Found') = 'No Update Found' and b.Gender <> '' then b.Gender
        when Coalesce(CLDG.CurrentValue, 'No Update Found') = 'No Update Found' and b.Gender = '' then 'No Update Found'
    end as GenderUpdate,

    Case 
        when Coalesce(nl.LOG_TEXT, 'No Update Found') <> 'No Update Found' then nl.Log_Text 
        When Coalesce(nl.LOG_TEXT, 'No Update Found') = 'No Update Found' and Cast(n.BIRTH_DATE as varchar) <> '' then Cast(n.BIRTH_DATE as VarChar)
        when Coalesce(nl.LOG_TEXT, 'No Update Found') = 'No Update Found' and Cast(n.BIRTH_DATE as varchar) = '' then 'No Update Found'
    end as BirthDateUpdate

from Customer as n inner join
    Members as b on b.ID = n.ID left join
    Profession as CLDP on n.ID = CLDP.ID left join
    Ethnicity as CLDE on n.ID = CLDE.ID Left Join
    Gender as CLDG on n.ID = CLDG.ID Left Join
    BirthDate as nl on nl.ID = n.ID 
Where n.ID in ('12345', '67890') and  
    cldp.createdon between @startDate and @endDate and 
    clde.createdon between @startdate and @enddate and 
    cldg.createdon between @startdate and @enddate and 
    nl.date_time between @StartDate and @EndDate

【问题讨论】：

你可以检查日期是否为空
那么在 were 语句中运行一个合并还是一个 IS NULL？在我今天运行的测试的情况下，查询结果没有数据，因为从 4 个表中提取的 3 个表中根本没有问题 ID。
这能回答你的问题吗？ Is it possible to return empty row from Sql Server?

标签： sql sql-server-2016

【解决方案1】：

您可以将outer apply 与包含单个虚拟行的左表一起使用。像下面这样：

select *
  into t
from(values
    (1, 'a'),
    (2, 'b'),
    (3, 'c')
) as t(id, val)

select t.*
from (
  select 1 as dummy
) as dummy
  outer apply (
    select *
    /*Your query goes here*/
    from t
    where id > 10
  ) as t

编号 |值 ---: | :--- 空 | 空

select t.*
from (
  select 1 as dummy
) as dummy
  outer apply (
    select *
    /*Your query goes here*/
    from t
    where id > 1
  ) as t

编号 |值 -: | :-- 2 | b 3 | C

db小提琴here

【讨论】：