Question

可以将以下查询转换为使用内部联接而不是where IN子查询吗？

select i.*, it.*
from ItemTypes it
    inner join Items i on (i.itemTypeID = it.ItemTypeID)
where i.itemID IN (......)

我可能在子查询中返回了100个结果，我想避免在子查询中使用它。

Answer 1

您可以使用WITH common table expression (CTE)：

;with SelectedItems (selectedIDs) as
(
    select distinct itemID from [...] where [...]
)
select i.*, it.*
from ItemTypes it
inner join Items i on i.itemTypeID = it.ItemTypeID
inner join SelectedItems on selectedIDs = i.itemID

您还可以定义一个带有索引的临时#Table来保存选择：

create table #SelectedItems
(
    selItemID int primary key clustered
)
insert into #SelectedItems (selItemID)
select [...]

select i.*, it.*
from ItemTypes it
inner join Items i on i.itemTypeID = it.ItemTypeID
inner join #SelectedItems si on si.selItemID = i.itemID

drop table #SelectedItems

但无论你做什么，都要小心测量任何改变之前和之后的表现。

Answer 2

select i.*, it.*
from ItemTypes it
    inner join Items i on (i.itemTypeID = it.ItemTypeID)
    inner join (select itemID from Items where ...) i2 ON i.itemID = i2.itemID

Answer 3

你可以像这样写

select i.*, it.*
from ItemTypes it
    inner join Items i on (i.itemTypeID = it.ItemTypeID)
    inner join (select distinct itemID from [...] where [...]) i2
        on (i.itemID = i2.itemID)

但我怀疑你会从中获益。毕竟，现代数据库倾向于为常见查询和子句找到最佳执行计划。但是，在这种情况下，可能需要额外的工作来使用distinct运算符来删除重复项

将IN子查询转换为内连接

3 个答案: