Replace empty cells with NULL values in large number of columns - sql

Question

I have SQL table that has a large number of columns. For some reason, some columns have empty cells instead of NULL cells. I would like to make all empty cells in all the columns to be NULL.
I know that the way to go for a single column is:
UPDATE your_table SET column = NULL WHERE column = ''
However, I am not sure how to execute a similar logic efficiently for all columns without having to write the column names one by one.
Thanks,

Answer 1

Run the following query:
SELECT 'UPDATE yourtable SET ' + name + ' = NULL WHERE ' + name + ' = '''';'
FROM syscolumns
WHERE id = object_id('yourtable')
AND isnullable = 1;
The output of this query will be a chunk of SQL script like this:
UPDATE yourtable SET column1 = NULL WHERE column1 = '';
UPDATE yourtable SET column2 = NULL WHERE column2 = '';
UPDATE yourtable SET column3 = NULL WHERE column3 = '';
-- etc...
Copy and paste that SQL script into a new query and run it to update all your columns.

Answer 2

You could do a query on syscolumns to get a list of columns, and use the results to construct your query.
select quotename(name) + ' = nullif (' + quotename(name)+ ','''')'
from syscolumns
where id = object_id('yourtable')
Additionally, if you write your query as
update yourtable
set
yourcolumn=nullif(yourcolumn, ''),
yourcolumn2=nullif(yourcolumn2, ''),
...
then you can do it in a single query without a where clause

Answer 3

I actually use Robert N's answer above daily when I'm importing flat file data sets, so I put it into a stored procedure that I could pass a table name to. It just populates a temp table with the update statements, then executes each row in the table.
USE [master]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author: LikeableBias
-- Create date: 2016-06-27
-- Description: Finds and NULLs all blank values in table where column allows nulls
-- =============================================
CREATE PROCEDURE [dbo].[sproc_NullBlanks]
#tablename NVARCHAR(MAX)
AS
BEGIN
SET NOCOUNT ON;
--------Insert update statements to temp table for execution
DECLARE #statements TABLE (statement NVARCHAR(MAX))
INSERT INTO #statements
( statement )
SELECT ('UPDATE '+#tablename+' SET [' + name + '] = NULL WHERE ' + name + ' = '''';')
FROM syscolumns
WHERE id = OBJECT_ID(#tablename)
AND isnullable = 1;
--------Open cursor, execute statements, then close cursor
DECLARE #statement NVARCHAR(MAX)
DECLARE cur CURSOR LOCAL FOR
SELECT statement FROM #statements
OPEN cur
FETCH NEXT FROM cur INTO #statement
WHILE ##FETCH_STATUS = 0 BEGIN
EXEC sys.sp_executesql #statement
FETCH NEXT FROM cur INTO #statement
END
CLOSE cur
DEALLOCATE cur
END
GO