how to drop constraints in sql server

DECLARE @EmpName NVARCHAR(100)
SELECT @EmpName = 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 = 'dbo'
    AND tables.name = EmpTable
    AND all_columns.name = 'EmpGuid'

EXEC ('ALTER TABLE EmpTable DROP CONSTRAINT ' + @EmpName)
Alter Table EmpTable  drop column EmpGuid