SQL Server日期時間子查詢轉換錯誤？

Question

無法理解爲什麼當我的子查詢正確過濾出錯誤的日期數據（用戶輸入的是真實查詢），但是當我將子查詢的結果（乾淨日期）轉換回where子句的日期時間時，查詢失敗。我列出了一個無法顯示的示例。花了很長時間在這個迄今爲止討厭的生活。

select 
date_test 
from 
(
select 
    date_test 
from 
(
    select 
    '01/01/1980' as date_test 
    union select 
    'a' 
) as qry_bad_date 
where 
    ISDATE(date_test) = 1 
) as qry_only_valid_date 
where 
cast(date_test as datetime) = '01/01/1980' 

Answer 1

如果您使用查詢分析器，進入查詢菜單，選擇「顯示估計的執行計劃」，或按CTRL + L。 Sql Server的查詢優化器已經決定，比較date_test到你指定的日期在食物鏈上屬於更高的。如果添加ISDATE檢查您的where子句中正常工作：

select date_test 
from (select date_test 
     from (select '1980/01/01' as date_test 
       union 
       select 'a' 
      ) as qry_bad_date 
     where ISDATE(date_test) = 1 
    ) as qry_only_valid_date 
where ISDATE(date_test) = 1 and cast(date_test as datetime) = '1980/01/01' 

如果使用臨時表或表變量來強制查詢執行單獨它也可以工作：

declare @dt1 table (date_test varchar(20)) 
declare @dt2 table (date_test varchar(20)) 
insert @dt1 select '1980/01/01' union select 'a' 
insert @dt2 select date_test from @dt1 where ISDATE(date_test) = 1 
select date_test 
from @dt2 
where cast(date_test as datetime) = '1980/01/01' 

Answer 2

貌似這個也起作用。感謝所有的幫助！

select 
    date_test 
from 
(
    select 
     case isdate(date_test) when 1 then CAST(date_test as datetime) else null end as date_test 
    from 
    (
     select 
      '1980/01/01' as date_test 
     union select 
      'a' 
    ) as qry_bad_date 
    where 
     ISDATE(date_test) = 1 
) as qry_only_valid_date 
where 
    cast(date_test as datetime) = '1980/01/01'