Replace empty cells with NULL values in large number of columns

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