SQL查询将列名更改为单列值

如何解决SQL查询将列名更改为单列值

我正在使用SQL Server。不知道是哪个版本，但仍受支持。 我有一个表，该表包含我感兴趣的3列。我需要更改“硬件”列中的列名称以及每个列的计数。现在，我有以下内容：

SELECT 
    COUNT(a.EAMacBook13) AS 'MacBook13',COUNT(a.EAMacBook16) AS 'MacBook16',COUNT(a.EAStandLaptop) AS 'StandLaptop' 
FROM 
    Employee AS e,EmpAttributes AS a 
WHERE 
    a.EAEmpID = e.EmpID AND e.EmpProjID = 1

相反，我需要类似的东西：

这可能吗？

解决方法

基本思想是取消计数。如果您的数据库支持横向联接（也称为cross apply）和values()，则可以执行以下操作：

select x.hardware,count(x.val) cnt
from employee as e
inner join empattributes as ea on ea.eaempid = e.empid 
cross join lateral (values 
    ('EAMacBook13',ea.EAMacBook13),('EAMacBook16',ea.EAMacBook16),('EAStandLaptop',ea.EAStandLaptop)
) x(hardware,val)
where e.empprojid = 1
group by x.hardware

根据您的数据库，横向连接可以使用不同的语法。某些数据库使用cross apply而不是cross join lateral。

values()也并非在所有地方都受支持。例如，在Oracle中，您可以将横向联接的短语设置为：

cross apply (
    select 'EAMacBook13' as hardware,EAMacBook13 as val from dual
    union all select 'EAMacBook16',EAMacBook16 from dual
    union all select 'EAStandLaptop',EAStandLaptop from dual
) x(hardware)

如果以上方法均不可用，那么我们可以退回到union all：

select x.hardware,count(x.val) cnt
from employee as e
inner join (
    select eaempid,'EAMacBook13' as hardware,EAMacBook13 as value from empattributes 
    union all select eaempid,'EAMacBook16',EAMacBook16 from empattributes 
    union all select eaempid,'EAStandLaptop',EAStandLaptop from empattributes 
) x on x.eaempid = e.empid 
where e.empprojid = 1
group by x.hardware

,

您可以通过简单的全部合并来实现：

SELECT 'MacBook13' "Hardware",Count(a.EAMacBook13) "Count"
FROM Employee as e,EmpAttributes as a WHERE a.EAEmpID = e.EmpID and e.EmpProjID = 1
union all 
SELECT 'MacBook16' "Hardware",Count(a.EAMacBook16)  "Count"
FROM Employee as e,EmpAttributes as a WHERE a.EAEmpID = e.EmpID and e.EmpProjID = 1
union all 
SELECT 'StandLaptop' "Hardware",Count(a.EAStandLaptop)  "Count"
FROM Employee as e,EmpAttributes as a WHERE a.EAEmpID = e.EmpID and e.EmpProjID = 1

根据您的RDBMS，您可能可以访问UNPIVOT，从而减少编写工作。

您还可以使用CTE，以便（如果RDBMS支持实现它们）您可以减少工作量：

with cte as (
  SELECT Count(a.EAMacBook13) as EAMacBook13,Count(a.EAMacBook16) as EAMacBook16,Count(a.EAStandLaptop) as EAStandLaptop
  FROM Employee as e,EmpAttributes as a WHERE a.EAEmpID = e.EmpID and e.EmpProjID = 1
)
SELECT 'MacBook13' "Hardware",EAMacBook13 "Count"
FROM  cte
union all 
SELECT 'MacBook16' "Hardware",MacBook16 "Count"
FROM cte
union all 
SELECT 'StandLaptop' "Hardware",StandLaptop "Count"
FROM cte

除非使用某种程度的动态SQL，否则您必须列出可能的硬件值。

,

一个选项是使用“动态取消旋转”。首先使用当前查询创建一个表：

SELECT a.EAMacBook13,a.EAMacBook16,a.EAStandLaptop
  INTO tab
  FROM Employee AS e 
  JOIN EmpAttributes AS a 
    ON a.EAEmpID = e.EmpID
 WHERE e.EmpProjID = 1

然后使用information_schema.columns：

DECLARE @cols  AS NVARCHAR(MAX),@query AS NVARCHAR(MAX)

SET  @query = 
    (SELECT STRING_AGG(CONCAT('SELECT ''',SUBSTRING(column_name,3,LEN(column_name)),''' AS Hardware,COUNT(',column_name,') AS Count FROM tab '),' UNION ALL ')
       FROM information_schema.columns
      WHERE table_name = 'tab');

EXEC sp_executesql @query;

Demo