Question

I have a SQL Server table which has 625 columns created with different datatypes like int, varchar(25), decimal(18, 2), etc…

Now am interested in changing datatype of all columns to varchar(255). Instead of executing below query for all the columns one by one, is there a single SQL Server query to change datatypes of all columns in a table at one shot?

ALTER TABLE dbo.Employee 
ALTER COLUMN FirstName VARCHAR(255) NOT NULL

Looking forward for your response.

Answer 1

There is no one single “magic” bullet to do this – it’s an operation that’s rather unusual, so it’s not supported natively.

What you can do is iterate over the columns of your table from the system catalog views, create such an ALTER statement on the fly, and also execute it – something like this:

DECLARE AlterTableCursor CURSOR FAST_FORWARD 
FOR
    SELECT 
        AlterCmd = 'ALTER TABLE dbo.YourTableNameHere ALTER COLUMN ' + name + ' VARCHAR(255) NULL'
    FROM 
        sys.columns
    WHERE 
        object_id = OBJECT_ID('dbo.YourTableNameHere')

DECLARE @AlterTableCmd NVARCHAR(200)

OPEN AlterTableCursor

FETCH NEXT FROM AlterTableCursor INTO @AlterTableCmd

WHILE @@FETCH_STATUS = 0
BEGIN
    EXEC (@AlterTableCmd)

    FETCH NEXT FROM AlterTableCursor INTO @AlterTableCmd
END

CLOSE AlterTableCursor
DEALLOCATE AlterTableCursor

Replace the YourTableNameHere with your actual table name – and you should be good to go! Test this on a copy of your live data first !