这种类型的转换称为枢轴。您没有指定您正在使用的数据库，因此我将为SQL Server和MySQL提供答案。

SQL Server：如果您使用的是SQL Server 2005，则可以实现PIVOT功能。

如果您要将已知数量的值转换为列，则可以对查询进行硬编码。

select typename, total, Deployed, Inventory, shipped

from

(

select count(*) over(partition by t.typename) total,

s.statusname,

t.typename

from assets a

inner join assettypes t

on a.assettype = t.id

inner join assetstatus s

on a.assetstatus = s.id

) d

pivot

(

count(statusname)

for statusname in (Deployed, Inventory, shipped)

) piv;

但是，如果您具有未知数量的状态值，则需要使用动态sql在运行时生成列列表。

DECLARE @cols AS NVARCHAR(MAX),

@query AS NVARCHAR(MAX)

select @cols = STUFF((SELECT distinct ',' + QUOTENAME(statusname)

from assetstatus

FOR XML PATH(''), TYPE

).value('.', 'NVARCHAR(MAX)')

,1,1,'')

set @query = 'SELECT typename, total,' + @cols + ' from

(

select count(*) over(partition by t.typename) total,

s.statusname,

t.typename

from assets a

inner join assettypes t

on a.assettype = t.id

inner join assetstatus s

on a.assetstatus = s.id

) x

pivot

(

count(statusname)

for statusname in (' + @cols + ')

) p '

execute(@query)

这也可以使用带有case表达式的聚合函数编写：

select typename,

total,

sum(case when statusname ='Deployed' then 1 else 0 end) Deployed,

sum(case when statusname ='Inventory' then 1 else 0 end) Inventory,

sum(case when statusname ='Shipped' then 1 else 0 end) Shipped

from

(

select count(*) over(partition by t.typename) total,

s.statusname,

t.typename

from assets a

inner join assettypes t

on a.assettype = t.id

inner join assetstatus s

on a.assetstatus = s.id

) d

group by typename, total

MySQL：这个数据库没有pivot函数，所以你必须使用aggregate函数和CASE表达式。它也没有窗口函数，因此您必须稍微更改查询到以下内容：

select typename,

total,

sum(case when statusname ='Deployed' then 1 else 0 end) Deployed,

sum(case when statusname ='Inventory' then 1 else 0 end) Inventory,

sum(case when statusname ='Shipped' then 1 else 0 end) Shipped

from

(

select t.typename,

(select count(*)

from assets a1

where a1.assettype = t.id

group by a1.assettype) total,

s.statusname

from assets a

inner join assettypes t

on a.assettype = t.id

inner join assetstatus s

on a.assetstatus = s.id

) d

group by typename, total;

然后，如果您需要MySQL中的动态解决方案，则必须使用预准备语句来生成要执行的sql字符串：

SET @sql = NULL;

SELECT

GROUP_CONCAT(DISTINCT

CONCAT(

'sum(CASE WHEN statusname = ''',

statusname,

''' THEN 1 else 0 END) AS `',

statusname, '`'

)

) INTO @sql

FROM assetstatus;

SET @sql

= CONCAT('SELECT typename,

total, ', @sql, '

from

(

select t.typename,

(select count(*)

from assets a1

where a1.assettype = t.id

group by a1.assettype) total,

s.statusname

from assets a

inner join assettypes t

on a.assettype = t.id

inner join assetstatus s

on a.assetstatus = s.id

) d

group by typename, total');

PREPARE stmt FROM @sql;

EXECUTE stmt;

DEALLOCATE PREPARE stmt;

两个数据库中的所有查询的结果都相同：

| TYPENAME | TOTAL | DEPLOYED | INVENTORY | SHIPPED |

-----------------------------------------------------

| Desktop | 2 | 1 | 1 | 0 |

| Laptop | 1 | 0 | 0 | 1 |

| Server | 1 | 1 | 0 | 0 |