Dropping an existing constraint in Microsoft SQL Server - sql-server

Question

I have tried to drop an existing constraint. Here's what I have:
The name of the constraint is: Data.Leraar.IsGeheim.DefaultValue
and the name of the table is: Data.Leraa.
This is the command I'm currently using:
IF EXISTS(SELECT 1 FROM sys.foreign_keys WHERE parent_object_id = OBJECT_ID(N'dbo.Leraar'))
BEGIN
ALTER TABLE Data.Leraar DROP CONSTRAINT [Data.Leraar.IsGeheim.DefaultValue]
END
If I execute this command, it says Command(s) completed successfully.
But the constraint still exists.
How should I change this command?
So you mean like this:
IF EXISTS(SELECT 1 FROM sys.foreign_keys WHERE parent_object_id = OBJECT_ID(N'dbo.Leraar'))
BEGIN
ALTER TABLE DataData.Leraar DROP FOREIGN KEY IsGeheim.DefaultValue;
END
But then I will get an error
Sorry, but I am using microsoft SQL
if I just do an simple:
ALTER TABLE [Data].[Leraar] DROP CONSTRAINT [Data.Leraar.IsGeheim.DefaultValue]
GO
it works. But I first want to check if it exists
If I do it like this:
IF exists (
SELECT *
FROM sys.default_constraints
WHERE
parent_object_id = OBJECT_ID(N'Data.Leraar') and
name = 'Data.Leraar.IsGeheim.DefaultValue'
)
ALTER TABLE Data.Leraar Drop Constraint Data.Leraar.IsGeheim.DefaultValue
GO
I get error on this line
ALTER TABLE Data.Leraar Drop Constraint Data.Leraar.IsGeheim.DefaultValue
after Data. I get error:
Msg 102, Level 15, State 1, Line 15 Incorrect syntax near '.'.
This works:
IF NOT EXISTS(SELECT 1 FROM sys.foreign_keys WHERE parent_object_id = OBJECT_ID(N'dbo.Leraar'))
BEGIN
ALTER TABLE [Data].[Leraar] ADD CONSTRAINT [Data.Leraar.IsGeheim.DefaultValue] DEFAULT ((1)) FOR [IsGeheim]
END
But to drop if it exists doens work
I have it now like this:
IF EXISTS (
SELECT *
FROM sys.default_constraints
WHERE
parent_object_id = OBJECT_ID(N'Data.Leraar') and
name = 'Data.Leraar.IsGeheim.DefaultValue'
)
ALTER TABLE Data.Leraar Drop Constraint [Data.Leraar.IsGeheim.DefaultValue]
PRINT 'constraint [Data.Leraar.IsGeheim.DefaultValue] has been dropped'
GO
IF NOT EXISTS((SELECT 1 FROM sys.foreign_keys WHERE parent_object_id = OBJECT_ID(N'dbo.Leraar') AND name ='Data.Leraar.IsGeheim.DefaultValue'))
BEGIN
ALTER TABLE [Data].[Leraar] ADD CONSTRAINT [Data.Leraar.IsGeheim.DefaultValue] DEFAULT ((1)) FOR [IsGeheim]
PRINT 'Constraint has been created [Data.Leraar.IsGeheim.DefaultValue]'
END
But is this correct?

Answer 1

You can also query sys.default_constraints system view as follows including the DROP Constraint clause of ALTER TABLE command
Please replace the table name and constraint name according to your case
IF exists (
SELECT *
FROM sys.default_constraints
WHERE
parent_object_id = OBJECT_ID(N'Table_1') and
name = 'DF_Table_1_col1'
)
ALTER TABLE Table_1 Drop Constraint DF_Table_1_col1
GO
I update the above command as follows since as I understand there is a misunderstanding
I assume now the schema for the table Leraar is Data
So please instead of dbo use Data as schema name
IF exists (
SELECT *
FROM sys.default_constraints
WHERE
parent_object_id = OBJECT_ID(N'Data.Leraar') and
name = 'Data.Leraar.IsGeheim.DefaultValue'
)
ALTER TABLE Data.Leraar Drop Constraint [Data.Leraar.IsGeheim.DefaultValue]
GO

Answer 2

You can simply use
IF (OBJECT_ID('Con_First') IS NOT NULL)
BEGIN
ALTER TABLE Customer DROP CONSTRAINT Con_First;
END
Here Con_First is the name of constraint you want to delete.