Mohammed Ali Mohammed Ali - 7 months ago 45
SQL Question

Drop default constraint on a column in TSQL

I have a table with a column like this that is currently live:

name NVARCHAR(128) NOT NULL DEFAULT ''


I am altering the column like this to make it nullable:

ALTER TABLE mytable ALTER COLUMN name NVARCHAR(128) NULL


However, the default constraint, named 'DF_mytable_datab__7DE4B36' in one instance of the table, still remains. I know this could have been avoided if the original author named the constraint. I have several of instances of these tables but I don't want to manually delete every constraint in every table I have. What is the easiest and most elegant way of dropping this default constraint on a column in Sql Server that I can uniformily apply to every instance of this table?

EDIT



This is the script that I ended up using:

DECLARE @table_id AS INT
DECLARE @name_column_id AS INT
DECLARE @sql nvarchar(255)

-- Find table id
SET @table_id = OBJECT_ID('mytable')

-- Find name column id
SELECT @name_column_id = column_id
FROM sys.columns
WHERE object_id = @table_id
AND name = 'name'

-- Remove default constraint from name column
SELECT @sql = 'ALTER TABLE mytable DROP CONSTRAINT ' + D.name
FROM sys.default_constraints AS D
WHERE D.parent_object_id = @table_id
AND D.parent_column_id = @name_column_id
EXECUTE sp_executesql @sql


Another script that can be used to accomplish this can be found here:
How to drop SQL default constraint without knowing its name?

Thanks,
Mohammed

Answer

This is how you would drop the constraint

ALTER TABLE <schema_name, sysname, dbo>.<table_name, sysname, table_name>
   DROP CONSTRAINT <default_constraint_name, sysname, default_constraint_name>
GO

With a script

-- t-sql scriptlet to drop all constraints on a table
DECLARE @database nvarchar(50)
DECLARE @table nvarchar(50)

set @database = 'dotnetnuke'
set @table = 'tabs'

DECLARE @sql nvarchar(255)
WHILE EXISTS(select * from INFORMATION_SCHEMA.TABLE_CONSTRAINTS where constraint_catalog = @database and table_name = @table)
BEGIN
    select    @sql = 'ALTER TABLE ' + @table + ' DROP CONSTRAINT ' + CONSTRAINT_NAME 
    from    INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
    where    constraint_catalog = @database and 
            table_name = @table
    exec    sp_executesql @sql
END

Credits go to Jon Galloway http://weblogs.asp.net/jgalloway/archive/2006/04/12/442616.aspx

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