altering DEFAULT constraint on column SQL Server - sql-server

Question

I have an SQL script for creating a table, i would like the default of all but a few columns to be "" the others require a integer default of 0
The following creates the table. Some columns are removed because there are lots
CREATE TABLE [dbo].[PMIPatients]
(
[PID] [varchar](30) NOT NULL,
[PatientFirstName] [varchar](30) NULL,
[PatientLastName] [varchar](30) NULL,
[PatientDOB] [varchar](30) NULL,
[PatientDoctor] [varchar](30) NULL,
[PatientDiet] [varchar](50) NULL,
[PatientFallRiskLevel] [int] NULL,
[BedId] [int] NULL,
[BedDisplayInfo] TEXT NOT NULL DEFAULT ''
CONSTRAINT [PK_HL7Patient] PRIMARY KEY CLUSTERED
([PID] ASC) WITH (PAD_INDEX = OFF,
STATISTICS_NORECOMPUTE = OFF,
IGNORE_DUP_KEY = OFF,
ALLOW_ROW_LOCKS = ON,
ALLOW_PAGE_LOCKS = ON)
ON [PRIMARY]
)
ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
I wish to set a different default on selected columns, the following code does not work as it says that there is already a default constraint set. So i assumne i have to drop the constraint first.
ALTER TABLE [dbo].[PMIPatients] ADD
DEFAULT ((0))
FOR [PatientFallRiskLevel]
http://www.w3schools.com/sql/sql_default.asp says the follow code should be able to drop the DEFAULT like this
ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT
but i get a syntax error on DEFAULT
How do i alter/drop the DEFAULT constraint of specific columns

Answer 1

When you add a default, you should use names for your constraints. This way you can later refer to those constraints by name.
ALTER TABLE [dbo].[PMIPatients] ADD CONSTRAINT [PatientFallRiskLevel_Default] DEFAULT ((0)) FOR PatientFallRiskLevel
Then you can drop it using:
ALTER TABLE [dbo].[PMIPatients] DROP CONSTRAINT [PatientFallRiskLevel_Default]

Answer 2

The syntax is:
ALTER TABLE [dbo].[PMIPatients] ADD CONSTRAINT NameYourConstraint DEFAULT ((0)) FOR [PatientFallRiskLevel]
ALTER TABLE [dbo].[PMIPatients] DROP CONSTRAINT NameYourConstraint

Answer 3

When you create a constraint without a name, the SQL Server automatically create internal name. It can be see in the [sys].[default_constraints].
For example:
DROP TABLE IF EXISTS [dbo].[StackOverflow];
CREATE TABLE [dbo].[StackOverflow]
(
[colA] INT
,[colB] INT DEFAULT(0)
,[colC] CHAR(1) DEFAULT('x')
);
GO
SELECT *
FROM [sys].[default_constraints]
WHERE [parent_object_id] = OBJECT_ID('[dbo].[StackOverflow]');
GO
You can use the name to drop the constraint.
If you have more of them, you can use one of the scripts below:
-- all suported editions
DECLARE #DynamicTSQLStataement NVARCHAR(MAX)
,#TableNameWithSchema SYSNAME;
SET #TableNameWithSchema = '[dbo].[StackOverflow]';
SELECT #DynamicTSQLStataement = STUFF
(
(
SELECT ';ALTER TABLE ' + #TableNameWithSchema + ' DROP CONSTRAINT ' + QUOTENAME([name])
FROM [sys].[default_constraints]
WHERE [parent_object_id] = OBJECT_ID('[dbo].[StackOverflow]')
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)')
,1
,1
,''
);
EXEC sp_executesql #DynamicTSQLStataement;
GO
or
-- SQL Server 2016 or later
DECLARE #DynamicTSQLStataement NVARCHAR(MAX)
,#TableNameWithSchema SYSNAME;
SET #TableNameWithSchema = '[dbo].[StackOverflow]';
SELECT #DynamicTSQLStataement = STRING_AGG('ALTER TABLE ' + #TableNameWithSchema + ' DROP CONSTRAINT ' + QUOTENAME([name]), ';')
FROM [sys].[default_constraints]
WHERE [parent_object_id] = OBJECT_ID('[dbo].[StackOverflow]');
EXEC sp_executesql #DynamicTSQLStataement;
GO