oracle 如何在另一个 SQL 语句中使用输出
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How to use the Output of one SQL Statement in another
提问by Display Name
I have the following statement:
我有以下声明：
select region_id from regions
where region_name = 'Europe'
I need the Output from this in the following statement where 'bla' is:
我需要以下语句中的输出，其中 'bla' 是：
select count(*) from countries
where region_id = 'bla'
How can I do that?
我怎样才能做到这一点？
回答by mcabral
回答by Blindy
Subqueries to the rescue!
子查询来救援！
select distinct *
from countries
where region_id=(select top 1 ir.region_id
from regions ir
where ir.region_name = 'Europe' )
Alternatively, you can use inand give it a list of items returned from your query.
或者，您可以使用in并为其提供从查询返回的项目列表。
回答by OMG Ponies
Using EXISTS:
使用存在：
SELECT c.*
FROM COUNTRIES c
WHERE EXISTS (SELECT NULL
FROM REGIONS r
WHERE r.region_id = c.region_id
AND r.region_name = 'Europe')
My preference is to use EXISTS rather than IN because:
我更喜欢使用 EXISTS 而不是 IN，因为：
INin Oracle has a limit of 1,000 valuesEXISTSallows you to match on more than one column if necessaryEXISTSreturns true on the first match, which can be faster than IN/etc depending on needs
IN在 Oracle 中有 1,000 个值的限制EXISTS允许您在必要时匹配多列EXISTS在第一次匹配时返回 true，根据需要，它可以比 IN/etc 更快
Most mistake EXISTSas a correlated subquery, but it executes differently & doesn't evaluate the SELECTclause - you can test using:
大多数错误都EXISTS作为相关子查询，但它的执行方式不同并且不评估SELECT子句 - 您可以使用以下方法进行测试：
SELECT c.*
FROM COUNTRIES c
WHERE EXISTS (SELECT 1/0
FROM REGIONS r
WHERE r.region_id = c.region_id
AND r.region_name = 'Europe')
