How to Drop a SQL Server Default Constraint Without Knowing Its Name?

Dropping SQL Default Constraints Without Knowing the Name

In SQL Server, default constraints are a way to specify a value that will be automatically assigned to a column if no other value is provided. However, there can be situations where the default constraint name is unknown or has been mistyped.

The Challenge

To drop a default constraint, the ALTER TABLE DROP CONSTRAINT syntax requires specifying the constraint name. Unfortunately, information about default constraints is not readily available through the INFORMATION_SCHEMA table.

The Solution

One approach to overcome this challenge is to use dynamic SQL to generate the command to drop the constraint based on the schema, table, and column names. Here's an example:

declare @schema_name nvarchar(256)
declare @table_name nvarchar(256)
declare @col_name nvarchar(256)
declare @Command  nvarchar(1000)

set @schema_name = N'MySchema'
set @table_name = N'Department'
set @col_name = N'ModifiedDate'

select @Command = 'ALTER TABLE ' + @schema_name + '.[' + @table_name + '] DROP CONSTRAINT ' + d.name
 from sys.tables t
  join sys.default_constraints d on d.parent_object_id = t.object_id
  join sys.columns c on c.object_id = t.object_id and c.column_id = d.parent_column_id
 where t.name = @table_name
  and t.schema_id = schema_id(@schema_name)
  and c.name = @col_name

execute (@Command)

This script generates the necessary ALTER TABLE DROP CONSTRAINT command dynamically based on the provided information and executes it, removing the default constraint without requiring its exact name.

By utilizing dynamic SQL, this method allows for the removal of default constraints without the need for prior knowledge or potential errors due to incorrect constraint names.

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