使用 SQL Server SELECT INTO 创建一个可为空的列? [英] Create a nullable column using SQL Server SELECT INTO?

问题描述

当我在 SQL Server 中使用 select into 创建临时表时，有没有办法指定列应该可以为空?我有一个多步骤的过程，我通过选择很多列来制作临时表(这就是为什么我不做 create table #tmp (...)).创建临时表后，我将更新一些列，其中一些更新可能会使某个字段无效.

When I create a temp table using a select into in SQL Server, is there a way to specify that a column should be nullable? I have a multi-step process where I'm making a temp table by selecting a lot of columns (which is why I'm not doing a create table #tmp (...)). After I make that temp table, I'm updating some columns and some of those updates might null out a field.

我知道我可以做一个 alter table alter column 语句来实现我想要的，但我很好奇是否有办法在 select 中指定它本身.我知道您可以内联 cast 列以获得所需的数据类型，但我看不到您如何指定可空性.

I know I could do an alter table alter column statement to achieve what I want, but I'm curious about whether there's a way to specify this in the select itself. I know you can inline cast your columns to get the desired datatype, but I can't see how you specify nullability.

推荐答案

Nullability 继承自源列.

Nullability is inherited from the source column.

您可以使用表达式失去或获得可空性:

You can lose or gain nullability with an expression:

示例(常量文字似乎有问题 - 需要一个可以返回 NULL 的良好的 NOOP 函数):

Example (constant literals appear to be problematic - need a good NOOP function which can return NULL):

CREATE TABLE SO5465245_IN
    (
     a INT NOT NULL
    ,b INT NULL
    ) ;
GO

SELECT  COALESCE(a, NULL) AS a
       ,ISNULL(b, 0) AS b
       ,COALESCE(10, NULL) AS c1
       ,COALESCE(ABS(10), NULL) AS c2
       ,CASE WHEN COALESCE(10, NULL) IS NOT NULL THEN COALESCE(10, NULL) ELSE NULL END AS c3
INTO    SO5465245_OUT
FROM    SO5465245_IN ;
GO

SELECT  TABLE_NAME
       ,COLUMN_NAME
       ,IS_NULLABLE
FROM    INFORMATION_SCHEMA.COLUMNS
WHERE   TABLE_NAME LIKE 'SO5465245%'
ORDER BY TABLE_NAME
       ,ORDINAL_POSITION ;
GO

DROP TABLE SO5465245_IN ;
GO

DROP TABLE SO5465245_OUT ;
GO

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