Question

以下代码从表中删除与非活动项目相关的任务记录。

delete from [Deliverables] where 
[Deliverables].[ProjectID] not in 
(
select 
[ProjectID] from [ActiveProjects]
)

我在某处读过，使用带有子查询的NOT IN返回很多值并不是最有效的事情，最好使用EXCEPT子句。

但是，当我尝试使用以下代码时，我收到错误（关键字'除'之外的语法不正确。）

delete from [Deliverables]
except
select * from [Deliverables], [ActiveProjects]
where [Deliverables].[ProjectID] = [ActiveProjects].[ProjectID]

如何将EXCEPT与DELETE一起使用？如果我不能，有没有办法优化我的查询以更快地执行？

Answer 1

你可以尝试not exists，脚本看起来像：

delete from [Deliverables] 
where not exists 
    (select 1 
        from [ActiveProjects]
        where [ActiveProjects].[ProjectID] = [Deliverables].[ProjectID])

如果[ActiveProjects]中有大量数据，那么它应该是更好的解决方案，但它是所有数据依赖的，所以请在使用前测试效率。

Answer 2

delete d
from [Deliverables] as d
    inner join (
        select d2.[ProjectId] from [Deliverables] as d2
        EXCEPT
        select ap.[ProjectId] from [ActiveProjects] as ap
    } as todel on
        ((todel.[ProjectId] is NULL) and (d.[ProjectId] is NULL))
        or todel.[ProjectId] = d.[ProjectId]

一个微妙之处：EXCEPT的优点是它将等同于NULL值，而不是确定它们永远不匹配（就像“ =”一样）。在您的琐碎示例（仅查看id）中，这不太可能有价值，但是，如果您要比较允许null的表的较大部分，则必须考虑这种尴尬的“ null匹配”，否则会遗漏很多行。

Answer 3

尝试这样（通过添加where-clauses和列名等来修改您的需求）

delete from table1
  from table1 a
 inner join
      ( select your_column
          from table1
        except
        select your_column
          from table2
      ) b
    on a.your_column = b.your_column;

使用T-SQL EXCEPT和DELETE /优化查询

3 个答案: