SQL Server中ORDER BY后面可以是表达式和子查询

假如SQL Server数据库中现在有Book表如下

CREATE TABLE [dbo].[Book](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [BookName] [nvarchar](50) NULL,
    [BookDescription] [nvarchar](50) NULL,
    [ISBN] [nvarchar](20) NULL,
    [CreateTime] [datetime] NULL,
 CONSTRAINT [PK_Book] PRIMARY KEY CLUSTERED
(
    [ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO

ALTER TABLE [dbo].[Book] ADD  CONSTRAINT [DF_Book_CreateTime]  DEFAULT (getdate()) FOR [CreateTime]

有如下数据：

SET IDENTITY_INSERT [dbo].[Book] ON
GO
INSERT [dbo].[Book] ([ID], [BookName], [BookDescription], [ISBN], [CreateTime]) VALUES (1, N'Chinese', N'This is a very good Chinese book', N'', CAST(N'2018-10-17T15:25:18.450' AS DateTime))
GO
INSERT [dbo].[Book] ([ID], [BookName], [BookDescription], [ISBN], [CreateTime]) VALUES (2, N'English', N'English', N'', CAST(N'2018-10-17T15:25:18.457' AS DateTime))
GO
INSERT [dbo].[Book] ([ID], [BookName], [BookDescription], [ISBN], [CreateTime]) VALUES (3, N'Japanese', N'Japanese', N'', CAST(N'2018-10-17T15:25:18.473' AS DateTime))
GO
INSERT [dbo].[Book] ([ID], [BookName], [BookDescription], [ISBN], [CreateTime]) VALUES (4, N'Russian', N'Russian', N'', CAST(N'2018-10-17T15:25:18.483' AS DateTime))
GO
INSERT [dbo].[Book] ([ID], [BookName], [BookDescription], [ISBN], [CreateTime]) VALUES (5, N'Italian', N'Italian', N'', CAST(N'2018-10-17T15:25:18.493' AS DateTime))
GO
SET IDENTITY_INSERT [dbo].[Book] OFF

我们使用SELECT语句查询该表，如下所示：

SELECT *
FROM [dbo].[Book]

现在设想一个问题，我们如何根据[BookName]和[BookDescription]两列数据的联合值来对结果进行排序呢？

我想很多人都会想到用子查询，如下所示：

SELECT [ID],[BookName],[BookDescription],[ISBN],[CreateTime]
FROM
(
    SELECT [ID],[BookName],[BookDescription],[ISBN],[CreateTime],[BookName]+N'#'+[BookDescription] AS [Combine]
    FROM [dbo].[Book]
) AS T
ORDER BY [Combine]

像上面这样用子查询的确没有问题，但是你知道吗，我们是可以直接在ORDER BY语句中写表达式的，如下所示：

SELECT *
FROM [dbo].[Book]
ORDER BY [BookName]+N'#'+[BookDescription]

结果和用子查询完全一样

我们也可以在ORDER BY中使用多个表达式和列来对结果进行排序，甚至我们可以根据一个子查询来对结果进行排序，如下所示：

SELECT *
FROM [dbo].[Book]
ORDER BY [BookName]+N'#'+[BookDescription] ASC,
         (SELECT TOP 1 R_BOOK.[ISBN] FROM [dbo].[Book] AS R_BOOK WHERE R_BOOK.[BookName]=[BookName]) DESC, --这里的子查询只能返回一行和一列数据，否则SQL Server会报错
         [CreateTime] ASC

但是ORDER BY后面不能是常量，比如下面这样我们在ORDER BY后面跟一个字符串常量是不行的：

SELECT *
FROM [dbo].[Book]
ORDER BY N'Constant'

执行该语句会报错：

Msg 408, Level 16, State 1, Line 3
A constant expression was encountered in the ORDER BY list, position 1.