计算列帮助-TSQL [英] Computed Column Help - TSQL

问题描述

CREATE TABLE [dbo].[tblLocations](
    [latitude] [float] NOT NULL,
    [longitude] [float] NOT NULL,
    [location] [varchar](500) NOT NULL,
    [timestamp] [datetime] NOT NULL,
    [point] [geography] AS geography::Point(latitude, longitude, 4326) NOT NULL
)

在 AS 。

这不是您声明计算列的方式吗？

Isn't this how you declare a computed column?

推荐答案

您自己无需声明数据类型或可为空

You don't declare the datatype or nullability yourself

CREATE TABLE [dbo].[tblLocations](
    [latitude] [float] NOT NULL,
    [longitude] [float] NOT NULL,
    [location] [varchar](500) NOT NULL,
    [timestamp] [datetime] NOT NULL,
    [point] AS geography::Point(latitude, longitude, 4326) 
)

通常，SQL Server将假定该列为可空除非您在公式周围添加 ISNULL（） 。

In general SQL Server will assume the column is nullable unless you add an ISNULL() around the formula.

但是我刚刚尝试了以下列定义

However I just tried the following column definition

[point2] AS ISNULL(geography::Point(latitude, longitude, 4326),
    geography::STGeomFromText('LINESTRING(-122.360 47.656, -122.343 47.656)', 4326))

，并且在 sys.computed_columns 中仍然显示为 is_nullable 看起来适用于CLR数据类型（可能是因为SQL Server不相信它们具有确定性）。

and that still shows up as is_nullable in sys.computed_columns so it doesn't look like that applies to CLR datatypes (probably as SQL Server doesn't trust these to be deterministic).

编辑：但是它 NOT NULL ，只要计算列被标记为 PERSISTED 即

However it is syntactically valid to specify NOT NULL as long as the computed column is marked as PERSISTED i.e.

[point] AS geography::Point(latitude, longitude, 4326) PERSISTED NOT NULL

在这种特殊情况下，尝试创建具有此类定义的表给出运行时错误。

In this particular case however attempting to create a table with such a definition gives a runtime error.

表
'foo'中的计算列'point'不能持久，因为
列类型地理是一种
非字节排序的CLR类型。

Computed column 'point' in table 'foo' cannot be persisted because the column type, 'geography', is a non-byte-ordered CLR type.

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