SQL Server 日期时间子查询转换错误？

Question

无法理解为什么当我的子查询正确过滤掉错误的日期数据（用户在实际查询中输入）但当我将子查询的结果（具有干净的日期）转换回日期时间时查询失败条款。我已经包含了一个失败的无表格示例。在这件事上花了很长时间——讨厌生活。

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

如果您使用的是查询分析器，请转到查询菜单并选择“显示估计的执行计划”，或按 CTRL+L。 Sql Server 的查询优化器已决定将 date_test 与您指定的日期进行比较属于食物链上的更高级别。如果您将 ISDATE 检查添加到您的 where 子句中，它可以正常工作：

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'

如果您使用临时表或表变量来强制查询单独执行，它也可以：

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

看起来这也有效。感谢大家的帮助！

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'