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[英]SQL How to select columns multiple times on one row based on value in other column
[英]SELECT multiple columns based on value of one column
我想基于一列的值选择特定的列。
当前,我要做的是获取特定列的值并使用它进行另一个查询。 像这样:
SELECT service_type FROM agreements WHERE id = 1;
我将值传递给PHP变量,然后根据结果进行另一个查询。
样例代码:
if (service == 'internet')
SELECT ipreq, latency, ... FROM agreements WHERE id = 3 AND service_type = 'internet';
else
SELECT rates, markup, ... FROM agreements WHERE id = 3 AND service_type = 'vpn';
有办法做到这一点而不必诉诸多个查询吗?
聚苯乙烯
一些列来自另一个表。 我只想根据列的值从特定的表中选择特定的列(在本例中为service_type
)
好吧,假设在应用程序的后续过程中,您同样对结果列进行踩踏,则可以尝试使用
SELECT * FROM
(
select service, ipreq, latency, ... FROM agreements WHERE id = 3
UNION ALL
select service, rates, markup, ... FROM agreements WHERE id = 3
) tmp
WHERE service = 'internet'
这将为id = 3
选择所有可能的条目的组合,然后选择service
所需的条目。
但是,这仅适用于所有字段具有相同的类型。
也许您还应该重命名字段以统一对它们的访问,例如
SELECT * FROM
(
select service, ipreq as field1, latency as field2, ... FROM agreements WHERE id = 3
UNION ALL
select service, rates as field1, markup as field2, ... FROM agreements WHERE id = 3
) tmp
WHERE service = 'internet'
如果您不想这样做,则需要在应用程序中按列索引访问它们。 字段类型仍然必须相同。
编辑:
这是更新的代码:
SELECT * FROM
(
SELECT doc.id as docid, service, ipreq, latency,... FROM agreements WHERE service = 1
UNION ALL
SELECT doc.id as docid, service, rates, markup,... FROM agreements WHERE service = 2
) as tmp
WHERE docid = 1
现在,此代码将选择与特定服务类型有关的所有字段。
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