Why does SQL Server keep creating a DF constraint?

SQL tricks to handle default constraints

When battling with SQL Server, it's essential to have the skill to control default constraints dynamically to prevent nasty surprises, like unnamed constraints. Here are key techniques:

The Art of Dropping Constraints Gracefully

Prevent fatal error blows by ensuring the constraint exists before attempting to drop it out of sight:

-- well, if the constraint officer asks ...
IF EXISTS(SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[DF_YourTable_YourColumn]') AND type = 'D')
BEGIN
    -- then our hands are clean!
    ALTER TABLE dbo.YourTable DROP CONSTRAINT DF_YourTable_YourColumn
END
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The Alchemist: Turning Tables

When it comes to transforming tables, always choose to give explicit names to steer clear of pesky unnamed constraints:

-- The great renaming spell
ALTER TABLE YourTable ADD YourColumn INT NOT NULL 
CONSTRAINT DF_YourTable_YourColumn DEFAULT 0;
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The Exorcist: Removing Constraints and Columns Together

It's all about precision; remove the haunted column and its lurking unnamed default constraint in one swift motion:

-- The ghostbuster script!
ALTER TABLE YourTable
DROP COLUMN YourColumn, 
CONSTRAINT DF_AutoName;
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The Magician: Handling Unforeseen Constraints

Utilize the power of dynamic SQL to vanish variable constraint names effortlessly:

-- The wizard's best-kept secret
DECLARE @ConstraintName nvarchar(256);
SELECT @ConstraintName = name 
FROM sys.default_constraints 
WHERE parent_object_id = object_id('YourTable')
AND col_name(parent_object_id, parent_column_id) = 'YourColumn';

IF @ConstraintName IS NOT NULL
   EXEC('ALTER TABLE YourTable DROP CONSTRAINT ' + @ConstraintName);
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Don't forget to adjust DF_AutoName with the actual default constraint's name to reveal the magic trick correctly.

Heavy-duty strategies & Best debugging practices

Scripting like a Hacker

Develop a resilient script to handle a wide range of scenarios. Your codes must be qualified enough to locate and dispatch constraints even without exact names, and muster a LIKE operator when necessary:

-- The Ninja Punch
DECLARE @ConstraintPattern NVARCHAR(256) = 'DF_%_YourColumn';
DECLARE @SqlCmd NVARCHAR(MAX) = '';

SELECT @SqlCmd += 'ALTER TABLE ' + OBJECT_NAME(parent_object_id) 
+ ' DROP CONSTRAINT ' + name + '; '
FROM sys.default_constraints
WHERE name LIKE @ConstraintPattern;

EXEC sp_executeSQL @SqlCmd;
-- Ninja, vanish!
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Code Red: Prepare for Emergencies

A good programmer always leaves an emergency exit when making changes. Always ensure backout scripts can be set in motion if things go awry:

-- Save our souls
DECLARE @BackoutScript NVARCHAR(MAX) = (SELECT --...)
-- If the apocalypse is here...
EXEC(@BackoutScript);
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For Future's Sake: Create Maintenance-friendly Scripts

Your future-self will thank you for creating maintenance-friendly code patterns. These not only aid in the easy identification of default constraints but also in their seamless modification:

-- Future-proofing 101
CONSTRAINT DF_TableName_ColumnName_DefaultValue
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