Drop a column with a default constraint in SQL Server (IF EXISTS) - sql

Question

I am writing a sql script for dropping column and a default constraint. The following script works fine but i like to know if it is a right way of doing it.
Can i drop a default constraint with a column in one statement instead of using two separate ones?
IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = OBJECT_ID(N'[DF_Employees_EmpID]') AND type = 'D')
BEGIN
ALTER TABLE [dbo].[Employees] DROP CONSTRAINT [DF_Employees_EmpID]
END
GO
BEGIN
ALTER TABLE [dbo].[Employees] DROP COLUMN [EmpID]
END

Answer 1

In SQL Server 2005 upwards you can drop both the constraint and the column in one statement.
The syntax is
ALTER TABLE [ database_name . [ schema_name ] . | schema_name . ] table_name
DROP { [ CONSTRAINT ] constraint_name | COLUMN column } [ ,...n ]
The emphasis is on [ ,...n ], indicating multiple terms.
NB! Since the terms are processed sequentially, if the column being dropped is part of the constraint being dropped, then the constraint must be the first term, followed by the column term.
In your example:
ALTER TABLE [dbo].[Employees] DROP CONSTRAINT [DF_Employees_EmpID], COLUMN [EmpID]
So your code would be:
IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = OBJECT_ID(N'[DF_Employees_EmpID]') AND type = 'D')
BEGIN
ALTER TABLE [dbo].[Employees] DROP CONSTRAINT [DF_Employees_EmpID], COLUMN [EmpID]
END
GO
In SQL Server 2016 they have introduced the IF EXISTS clause which removes the need to check for the existence of the constraint first e.g.
ALTER TABLE [dbo].[Employees] DROP CONSTRAINT IF EXISTS [DF_Employees_EmpID], COLUMN IF EXISTS [EmpID]

Answer 2

Here is another way to drop a column & default constraints with checking if they exist before dropping them:
-------------------------------------------------------------------------
-- Drop COLUMN
-- Name of Column: Column_EmployeeName
-- Name of Table: table_Emplyee
--------------------------------------------------------------------------
IF EXISTS (
SELECT 1
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'table_Emplyee'
AND COLUMN_NAME = 'Column_EmployeeName'
)
BEGIN
IF EXISTS ( SELECT 1
FROM sys.default_constraints
WHERE object_id = OBJECT_ID('[dbo].[DF_table_Emplyee_Column_EmployeeName]')
AND parent_object_id = OBJECT_ID('[dbo].[table_Emplyee]')
)
BEGIN
------ DROP Contraint
ALTER TABLE [dbo].[table_Emplyee] DROP CONSTRAINT [DF_table_Emplyee_Column_EmployeeName]
PRINT '[DF_table_Emplyee_Column_EmployeeName] was dropped'
END
-- ----- DROP Column -----------------------------------------------------------------
ALTER TABLE [dbo].table_Emplyee
DROP COLUMN Column_EmployeeName
PRINT 'Column Column_EmployeeName in images table was dropped'
END
--------------------------------------------------------------------------
-- ADD COLUMN Column_EmployeeName IN table_Emplyee table
--------------------------------------------------------------------------
IF NOT EXISTS (
SELECT 1
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'table_Emplyee'
AND COLUMN_NAME = 'Column_EmployeeName'
)
BEGIN
----- ADD Column & Contraint
ALTER TABLE dbo.table_Emplyee
ADD Column_EmployeeName BIT NOT NULL
CONSTRAINT [DF_table_Emplyee_Column_EmployeeName] DEFAULT (0)
PRINT 'Column [DF_table_Emplyee_Column_EmployeeName] in table_Emplyee table was Added'
PRINT 'Contraint [DF_table_Emplyee_Column_EmployeeName] was Added'
END
GO

Answer 3

How you have it is fine.
An alternative would be
IF OBJECT_ID('DF_Employees_EmpID', 'D') IS NULL
BEGIN
ALTER TABLE dbo.Employees
DROP COLUMN EmpID
END
ELSE
BEGIN
ALTER TABLE dbo.Employees
DROP CONSTRAINT DF_Employees_EmpID,
COLUMN EmpID
END
In the event that the constraint does exist this combines the two operations into a single statement/transaction.

Answer 4

Another solution:
DECLARE #TableName sysname,
#Schema sysname,
#colname sysname,
#sql VARCHAR(1000)
SELECT #Schema = 'dbo',
#TableName = 'mytable',
#colname = 'mycol'
IF COL_LENGTH(#Schema+'.'+#TableName, #colname) IS NULL
BEGIN
PRINT 'Column does not exist!'
END
ELSE
BEGIN
SET #sql = ''
SELECT #sql += N' ALTER TABLE ' + #TableName + ' DROP CONSTRAINT ' + default_constraints.name + ';'
FROM sys.all_columns
INNER JOIN sys.tables
ON all_columns.object_id = TABLES.object_id
INNER JOIN sys.schemas
ON TABLES.schema_id = schemas.schema_id
INNER JOIN sys.default_constraints
ON all_columns.default_object_id = default_constraints.object_id
WHERE schemas.name = #Schema
AND tables.name = #TableName
AND all_columns.name = #colname
SET #sql += N' ALTER TABLE ' + #TableName + ' DROP COLUMN ' + #colname + ';'
PRINT ISNULL(#sql, 'NULL')
EXECUTE(#sql)
END