How to Drop SQL Server Default Constraints Without Knowing Their Names?

Deleting SQL Default Constraints Without Knowing Their Names

Background

SQL Server allows for the creation of default constraints on columns. However, in early versions, typos often resulted in inconsistent constraint names, such as "DF_SomeTable_ColName" and "DF_SmoeTable_ColName." This poses a challenge when attempting to drop a default constraint without knowing its precise name.

Solution

To effectively remove a default constraint without knowing its name, the following approach can be used:

1. Dynamically Generate Drop Command: Utilizing the combination of sys.tables, sys.default_constraints, and sys.columns tables, a dynamic string representing the SQL command to drop the constraint can be constructed. This string identifies the table, column, and specific default constraint to be removed.
2. Execute Dynamic Command: Once the drop command string has been generated, it can be executed dynamically using execute (@Command). This step executes the command and removes the default constraint.

Example Code

The following code sample provides an implementation of the described approach:

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

--print @Command

execute (@Command)

By utilizing this technique, default constraints can be removed dynamically, regardless of typo-induced name inconsistencies. This approach ensures that constraints are dropped successfully without encountering errors due to unknown constraint names.

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