在sql server中有效地将行转换为列
我正在寻找一种在SQL server中将行转换为列的有效方法，听说PIVOT不是很快，我需要处理很多记录.
I'm looking for an efficient way to convert rows to columns in SQL server, I heard that PIVOT is not very fast, and I need to deal with lot of records.
这是我的例子:
-------------------------------
| Id | Value | ColumnName |
-------------------------------
| 1 | John | FirstName |
| 2 | 2.4 | Amount |
| 3 | ZH1E4A | PostalCode |
| 4 | Fork | LastName |
| 5 | 857685 | AccountNumber |
-------------------------------
这是我的结果:
---------------------------------------------------------------------
| FirstName |Amount| PostalCode | LastName | AccountNumber |
---------------------------------------------------------------------
| John | 2.4 | ZH1E4A | Fork | 857685 |
---------------------------------------------------------------------
如何构建结果?
推荐答案
有几种方法可以将数据从多行转换为列.
There are several ways that you can transform data from multiple rows into columns.
在 SQL Server 中，您可以使用 PIVOT 函数将数据从行转换为列:
In SQL Server you can use the PIVOT function to transform the data from rows to columns:
select Firstname, Amount, PostalCode, LastName, AccountNumber
from
(
select value, columnname
from yourtable
) d
pivot
(
max(value)
for columnname in (Firstname, Amount, PostalCode, LastName, AccountNumber)
) piv;
参见演示.
如果您想要转置的 columnnames 数量未知，则可以使用动态 SQL:
If you have an unknown number of columnnames that you want to transpose, then you can use dynamic SQL:
DECLARE @cols AS NVARCHAR(MAX),
@query AS NVARCHAR(MAX)
select @cols = STUFF((SELECT ',' + QUOTENAME(ColumnName)
from yourtable
group by ColumnName, id
order by id
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)')
,1,1,'')
set @query = N'SELECT ' + @cols + N' from
(
select value, ColumnName
from yourtable
) x
pivot
(
max(value)
for ColumnName in (' + @cols + N')
) p '
exec sp_executesql @query;
参见演示.
如果您不想使用 PIVOT 函数，则可以使用带有 CASE 表达式的聚合函数:
If you do not want to use the PIVOT function, then you can use an aggregate function with a CASE expression:
select
max(case when columnname = 'FirstName' then value end) Firstname,
max(case when columnname = 'Amount' then value end) Amount,
max(case when columnname = 'PostalCode' then value end) PostalCode,
max(case when columnname = 'LastName' then value end) LastName,
max(case when columnname = 'AccountNumber' then value end) AccountNumber
from yourtable
参见演示.
这也可以使用多个连接来完成，但是您需要一些列来关联示例数据中没有的每一行.但基本语法是:
This could also be completed using multiple joins, but you will need some column to associate each of the rows which you do not have in your sample data. But the basic syntax would be:
select fn.value as FirstName,
a.value as Amount,
pc.value as PostalCode,
ln.value as LastName,
an.value as AccountNumber
from yourtable fn
left join yourtable a
on fn.somecol = a.somecol
and a.columnname = 'Amount'
left join yourtable pc
on fn.somecol = pc.somecol
and pc.columnname = 'PostalCode'
left join yourtable ln
on fn.somecol = ln.somecol
and ln.columnname = 'LastName'
left join yourtable an
on fn.somecol = an.somecol
and an.columnname = 'AccountNumber'
where fn.columnname = 'Firstname'
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