Question

我一直在尝试根据SQL Server docs编写子查询。我正在尝试将SQL Server查询写入

将varchar转换为int
选择varchar为null的行。

我的逻辑是：

outer select + where子句获取新列（q5int）为NULL的所有行
创建执行强制转换的内部选择并创建新列（q5int）

我做错了什么？我很感激我的逻辑错误解释，而不仅仅是如何修复我的代码。

select *  
from 
    (select 
         *, cast (NULLIF(q5,'NULL') as int) as q5int 
     from 
         ft_merge)
where
    q5int = NULL

Answer 1

您可以执行此操作无子查询

select *  
 from ft_merge
 WHERE try_convert(int,q5) is null

Answer 2

让我们先分析您的查询

               select *  from (
                select *,cast (NULLIF(q5,'NULL') as int) as q5int 
                from ft_merge
                ) WHERE q5int=NULL

1.NULLIF（q5，'NULL'）应该是NULLIF（q5，NULL）而不是引号

2.WHERE q5int = NULL应替换为“q5int IS NULL”

创建测试数据

       Select * into #ft_merge
        from
        (

              Select 1 AS ID,'111' AS q5
              union
              Select 2, null
                  union
              Select 3,'2222'
                  union
              Select 4,null 
        )t



       /* using your query */  
       select *  from (
      select *,cast(q5 as int) as q5int 
        from #ft_merge where ISNUMERIC(q5) > 0 or q5 is null
      )t WHERE q5int is NULL
      /* another optimal way of doing this */

      select *,cast(q5 as int) as q5int 
      from #ft_merge where NULLIF(q5,NULL) is null

子查询引用新创建的列

2 个答案: