How to Drop SQL Default Constraints Without Knowing Their Names?

Dropping SQL Default Constraints Without Knowing Their Names

In SQL Server, identifying and dropping default constraints can be challenging, especially when their names are unknown or inconsistent. To address this, dynamic query generation can be employed to identify and remove default constraints effectively.

Expanded Answer:

Expanding on the script shared by Mitch Wheat, a more comprehensive approach involves dynamically generating and executing the DROP CONSTRAINT command. Here's how it works:

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)

Explanation:

• This script dynamically builds the DROP CONSTRAINT command by querying the metadata tables (sys.tables, sys.default_constraints, and sys.columns).
• It specifies the schema name, table name, and column name to identify the target default constraint.
• The generated command is then printed and executed dynamically, allowing you to drop the default constraint without knowing its exact name.

Benefits:

• Handles inconsistently named constraints, such as "DF_SomeTable_ColName" or "DF_SmoeTable_ColName."
• Dynamically identifies and drops default constraints based on the specified table and column information.
• Avoids SQL errors by dynamically generating the correct command.

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