Question

我正在创建一个示例查询，它将行转换为列，如下所示：

Person_Id   Total Earned Leave  Earned Leave Enjoyed    Remaining Earned Leave  Total Casual Leave  Casual Leave Enjoyed    Remaining Casual Leave
1001        20                  10                      10                      20                  4                       16

以上是我得到的输出并使用以下查询使用多个子查询：

SELECT DISTINCT m.Person_Id, (SELECT k.Leave_Allocation FROM LeaveDetails k WHERE k.Leave_Name = 'Earn Leave' 
AND k.Person_Id = 1001 AND k.[Year] = '2017') AS 'Total Earned Leave',

(SELECT o.Leave_Enjoy FROM LeaveDetails o WHERE o.Leave_Name = 'Earn Leave' 
AND o.Person_Id = 1001 AND o.[Year] = '2017') AS 'Earned Leave Enjoyed',

(SELECT p.Leave_Remain FROM LeaveDetails p WHERE p.Leave_Name = 'Earn Leave' 
AND p.Person_Id = 1001 AND p.[Year] = '2017') AS 'Remaining Earned Leave',

(SELECT k.Leave_Allocation FROM LeaveDetails k WHERE k.Leave_Name = 'Casual Leave' 
AND k.Person_Id = 1001 AND k.[Year] = '2017') AS 'Total Casual Leave',

(SELECT o.Leave_Enjoy FROM LeaveDetails o WHERE o.Leave_Name = 'Casual Leave' 
AND o.Person_Id = 1001 AND o.[Year] = '2017') AS 'Casual Leave Enjoyed',

(SELECT p.Leave_Remain FROM LeaveDetails p WHERE p.Leave_Name = 'Casual Leave' 
AND p.Person_Id = 1001 AND p.[Year] = '2017') AS 'Remaining Casual Leave'

FROM LeaveDetails m WHERE m.Person_Id = 1001 AND m.[Year] = '2017'

我不确定我是否会遇到性能问题，因为会有大量数据，并且认为这比 Pivot 或运行时表创建更好< / strong>即可。我只是想确定这是否是我想要达到的目的的更好选择。您可以使用SQL Server，MySQL或Oracle分享您的想法和示例，以获得更好的性能问题 - 谢谢。

样本表和数据：

CREATE TABLE [dbo].[LeaveDetails]( [Id] [int] IDENTITY(1,1) NOT NULL, [Person_Id] [nvarchar](20) NULL, [Leave_Name] [nvarchar](40) NULL, [Leave_Allocation] [float] NULL, [Leave_Enjoy] [float] NULL, [Leave_Remain] [float] NULL, [Details] [nvarchar](100) NULL, [Year] [nvarchar](10) NULL, [Status] [bit] NULL ) INSERT [dbo].[LeaveDetails] ([Id], [Person_Id], [Leave_Name], [Leave_Allocation], [Leave_Enjoy], [Leave_Remain], [Details], [Year], [Status]) VALUES (1, N'1001', N'Earn Leave', 20, 10, 10, NULL, N'2017', 1) INSERT [dbo].[LeaveDetails] ([Id], [Person_Id], [Leave_Name], [Leave_Allocation], [Leave_Enjoy], [Leave_Remain], [Details], [Year], [Status]) VALUES (2, N'1001', N'Casual Leave', 20, 4, 16, NULL, N'2017', 1)

Answer 1

使用条件聚合：

SELECT m.Person_Id,
       MAX(CASE WHEN m.Leave_Name = 'Earn Leave' THEN k.Leave_Allocation END) as [Total Earned Leave],
       MAX(CASE WHEN m.Leave_Name = 'Earn Leave' THEN m.Leave_Enjoy END) as [Earned Leave Enjoyed],
       MAX(CASE WHEN m.Leave_Name = 'Earn Leave' THEN m.Leave_Remain END) as [Remaining Earned Leave],
       MAX(CASE WHEN m.Leave_Name = 'Casual Leave' THEN k.Leave_Allocation END) as [Total Casual Leave],
       MAX(CASE WHEN m.Leave_Name = 'Casual Leave' THEN k.Leave_Remain END) as [Casual Leave Enjoyed],
       MAX(CASE WHEN m.Leave_Name = 'Casual Leave' THEN k.Leave_Remain END) as [Remaining Casual Leave]
FROM LeaveDetails m
WHERE m.Person_Id = 1001 AND m.[Year] = '2017'
GROUP BY m.Person_ID;

注意：我不主张在列别名中包含特殊字符（例如空格）。如果这样做，请使用正确的转义字符（方括号）。仅对字符串和日期常量使用单引号。

Answer 2

PIVOT可以工作，但看起来这只是一个你想要转移到柱状输出的行，并且列名是明确知道的。如果是这种情况，您可以将UNION单列结果放在一起：

SELECT 'Person_ID' as col_name, Person_Id as col_value FROM LeaveDetails WHERE Person_Id = 1001 AND [Year] = '2017'
UNION
SELECT 'Leave_Enjoy' as col_name, Leave_Enjoy as col_value FROM LeaveDetails WHERE Person_Id = 1001 AND [Year] = '2017'
UNION
...

编写起来要简单得多，阅读起来更干净，并且应该运行得更快 - 每列仍然有一个表扫描。该表是否在Person_ID和Year？

上编制索引

如果速度是个问题，您可以创建一行的临时表：

SELECT * into #ld_temp FROM LeaveDetails WHERE Person_Id = 1001 AND [Year] = '2017'

然后从SELECT / UNION代码中的临时表中选择：

SELECT 'Person_ID' as col_name, Person_Id as col_value FROM #ld_temp
UNION
SELECT 'Leave_Enjoy' as col_name, Leave_Enjoy as col_value FROM #ld_temp
UNION
...

现在你只需要对大表进行一次扫描。

我希望这会有所帮助。

SQL查询中的多个子查询

2 个答案: