SQL DB2：不允许“NULL”列？
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DB2: Won't Allow "NULL" column?
提问by GyRo
Part of a complex query that our app is running contains the lines: ...(inner query)
我们的应用程序正在运行的复杂查询的一部分包含以下行：...（内部查询）
SELECT
...
NULL as column_A,
NULL as column_B,
...
FROM
...
This syntax of creating columns with nullvalues is not allowed in DB2 altough it is totally OK in MSSQLand OracleDBs. Technically I can change it to:
这种创建空值列的语法在 DB2 中是不允许的，尽管它在MSSQL和OracleDB 中完全可以。从技术上讲，我可以将其更改为：
'' as column_A,
'' as column_B,
But this doesn't have exactly the same meaning and can damage our calculation results. How can I create columns with null values in DB2 using other syntax??
但这并不具有完全相同的含义，并且会破坏我们的计算结果。如何使用其他语法在 DB2 中创建具有空值的列？
回答by Ian Bjorhovde
DB2 is strongly typed, so you need to tell DB2 what kind of column your NULL is:
DB2 是强类型的，因此您需要告诉 DB2 您的 NULL 是哪种列：
select
...
cast(NULL as int) as column_A,
cast(NULL as varchar(128)) as column_B,
...
FROM
...
回答by Paul Vernon
For Db2 LUW, since version 9.7, you can (if you like) use the NULLvalue without explicitly casting it to a particular data type.
对于 Db2 LUW，从 9.7 版开始，您可以（如果愿意）使用该NULL值，而无需将其显式转换为特定数据类型。
NULLon it's own will be implicitly cast to VARCHAR(1). This knowledge center page will show what happens in other cases: Determining data types of untyped expressions
NULL它自己将被隐式转换为VARCHAR(1). 此知识中心页面将显示在其他情况下会发生什么： 确定无类型表达式的数据类型
Examples
例子
db2 "describe values ( NULL, + NULL, NULL || NULL )"
Column Information
Number of columns: 3
SQL type Type length Column name Name length
-------------------- ----------- ------------------------------ -----------
449 VARCHAR 1 1 1
997 DECFLOAT 16 2 1
449 VARCHAR 508 3 1
and
和
db2 "describe values NULL, 1"
Column Information
Number of columns: 1
SQL type Type length Column name Name length
-------------------- ----------- ------------------------------ -----------
497 INTEGER 4 1 1
