Why is IS NOT NULL returning NULL values for a Varchar(max) in SQL Server?
sql-server, sql-server-2005
Solution
I got a sample code to reproduce the above behavior. The problem arises when you have a `TEXT` field that stores a value larger than it can fit in a row and if you set it afterwards to `NULL` and perform the column conversion to `VARCHAR(MAX)`.
The large value gets stored in a separate page. Then you set the value of this field to `NULL`. If you now convert this column to a `VARCHAR(MAX)`, then SQL Server seems to not get it right. Typically on a `TEXT` to `VARCHAR(MAX)` conversion the external pages stay as they are, but maybe because it was set to `NULL`, the column altering messes things up.
Update: It doesn't seem to have anything to do with the large values in the `TEXT` column. Short values show the same behavior (extended sample). So it's just the explicit setting to `NULL` through an `UPDATE` and the conversion that matters.
CREATE TABLE [dbo].[Test](
[Id] [int] NOT NULL,
[Value] [text] NULL,
CONSTRAINT [PK_Test] 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] TEXTIMAGE_ON [PRIMARY]
GO
INSERT INTO Test VALUES (1, 'test')
INSERT INTO Test VALUES (2, '')
INSERT INTO Test VALUES (3, NULL)
INSERT INTO Test VALUES (4, '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678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INSERT INTO Test VALUES (5, 'short string')
GO
update test SET value = null where ID = 4
update test SET value = null where ID = 5
GO
ALTER TABLE test ALTER COLUMN value varchar(max)
GO
select id, value, len(value) as length
from test
where value is not null
GO
The result is:
1 test 4
2 0
4 NULL NULL
5 NULL NULL
An easy fix for this problem would be to reassign the values in the `VARCHAR(MAX)` columns.
UPDATE Test SET value = value
This seems to put the values in the rows that were previously stored in external pages. (See for reference: NTEXT vs NVARCHAR(MAX) in SQL 2005)
Problem
- It looks like some NULL values are appearing in the list. - Some NULL values are being filtered out by the query. I have checked. - If I add `AND AdditionalFields = ''`, both these results are still returned - AdditionalFields is a varchar(max) - The database is SQL Server 10 with Compatibility Level = Sql Server 2005 (90) - I am using Management Studio 2008 I appear to have empty strings whose length is NULL, or NULL values that are equal to an empty string. Is this a new datatype?! EDIT: New datatype - hereby to be referred to as a "Numpty" EDIT 2 inserting the data into a temporary table turns Numpties into NULLS. (The result from this sql is 10) ``` CREATE TABLE #temp(ID uniqueidentifier , Value varchar(max)) INSERT INTO #temp SELECT top 10 g.ID, g.AdditionalFields FROM grants g WHERE g.AdditionalFields IS NOT NULL AND LEN(g.AdditionalFields) IS NULL SELECT COUNT(*) FROM #temp WHERE Value is null DROP TABLE #temp ``` EDIT 3 And I can fix the data by running an update: ``` UPDATE Grants SET AdditionalFields = NULL WHERE AdditionalFields IS NOT NULL AND LEN(AdditionalFields) IS NULL ``` So that makes me think the fields must contain something, rather than some problem with the schema definition. But what is it? And how do I stop it ever coming back? EDIT 4 There are 2 other fields in my database, both varchar(max) that return rows when the field IS NOT NULL AND LEN(field) IS NULL. All these fields were once TEXT and were changed to VARCHAR(MAX). The database was also moved from Sql Server 2005 to 2008. It looks like we've got ANSI_PADDING etc OFF by default. Another example: Converting to varbinary Execution plan: EDIT 5: removed table definition - turned out to be not relevant in the end EDIT 6 Scripts to generate scripts for altering TEXT to VARCHAR(MAX) then update values to prevent bug and enhance performance ``` --Generate scripts to alter TEXT to VARCHAR(MAX) SELECT 'ALTER TABLE [' + tab.table_schema + '].[' + tab.table_name + '] ALTER COLUMN [' + col.column_name + '] VARCHAR(MAX)' + CASE WHEN col.IS_NULLABLE = 'YES' THEN ' NULL' ELSE ' NOT NULL' END + ' GO' FROM INFORMATION_SCHEMA.tables tab INNER JOIN INFORMATION_SCHEMA.COLUMNS col ON col.table_name = tab.table_name AND tab.table_schema = col.table_schema AND tab.table_catalog = col.table_catalog WHERE tab.table_type <> 'VIEW' and col.DATA_TYPE = 'text' --Generate scripts to set value to value in VARCHAR(MAX) fields SELECT 'UPDATE [' + tab.table_schema + '].[' + tab.table_name + '] SET [' + col.column_name + '] = [' + col.column_name + ']' FROM INFORMATION_SCHEMA.tables tab INNER JOIN INFORMATION_SCHEMA.COLUMNS col ON col.table_name = tab.table_name AND tab.table_schema = col.table_schema AND tab.table_catalog = col.table_catalog WHERE tab.table_type <> 'VIEW' AND col.DATA_TYPE = 'varchar' and col.CHARACTER_MAXIMUM_LENGTH = -1 ```