sql - 将大量列中的空单元格替换为 NULL 值

Question

我有包含大量列的 SQL 表。出于某种原因，某些列有空单元格而不是 NULL 单元格。我想让所有列中的所有空单元格都为 NULL。

我知道单列的方法是：

 UPDATE your_table SET column = NULL WHERE column = ''

但是，我不确定如何为所有列有效地执行类似的逻辑，而不必一一编写列名。

谢谢，

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

您可以进行查询syscolumns以获取列列表，并使用结果来构建您的查询。

select quotename(name) + ' = nullif (' + quotename(name)+ ','''')'
from syscolumns 
where id = object_id('yourtable')

此外，如果您将查询编写为

update yourtable
set
    yourcolumn=nullif(yourcolumn, ''),
    yourcolumn2=nullif(yourcolumn2, ''),
    ...    

那么您可以在没有 where 子句的单个查询中执行此操作

Answer 3

实际上，当我导入平面文件数据集时，我每天都会使用上面 Robert N 的答案，所以我将它放入一个存储过程中，我可以将表名传递给它。它只是用更新语句填充临时表，然后执行表中的每一行。

    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