同一表的列上的SQL联接

具有此表（顶行为列名）：

需要查询以获得这些结果：

security    bid     ask
sec_1       3.4     3.6
sec_2       5.2     5.4
sec_3       2.3     null
sec_4       null    7.8

使用SQL Server

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谢谢你的帮助

您可以在表上的两个查询之间联接：

SELECT          a.security, bid, ask
FROM            (SELECT security, price AS bid
                 FROM   mytable
                 WHERE  quote_type = 'bid') a
FULL OUTER JOIN (SELECT security, price AS ask
                 FROM   mytable
                 WHERE  quote_type = 'ask') b ON a.security = b.security

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您也可以只使用条件聚合或透视：

select security,
       max(case when quote_type = 'bid' then price end) as bid,
       max(case when quote_type = 'ask' then price end) as ask
from t
group by security;

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如果每个证券将有超过1个出价怎么办？对于每个证券只有一对

bid

，

ask

，您可以使用条件聚合来获得预期结果。你可以在这里找到很多类似的案例。每个证券最多有一次出价和一次询问

select security,
       max(case when quote_type = 'bid' then price end) as bid,
       max(case when quote_type = 'ask' then price end) as ask
from t
group by security;