alter_column_mssql.sql

-- ##mssql
create TABLE TEST_SEB (
  COL_1 nvarchar(10),
  COL_2 nvarchar(10) NOT NULL
)
-- do not change nullity nor type
alter table TEST_SEB ALTER COLUMN COL_1 nvarchar(10) NULL;
alter table TEST_SEB ALTER COLUMN COL_2 nvarchar(10) NOT NULL;

-- change nullity
alter table TEST_SEB ALTER COLUMN COL_1 nvarchar(10) NOT NULL;
alter table TEST_SEB ALTER COLUMN COL_2 nvarchar(10) NULL;

-- change type
alter table TEST_SEB ALTER COLUMN COL_1 nvarchar(20) NOT NULL;
alter table TEST_SEB ALTER COLUMN COL_2 nvarchar(20) NULL;

-- change type and nullity
alter table TEST_SEB ALTER COLUMN COL_1 nvarchar(30) NULL;
alter table TEST_SEB ALTER COLUMN COL_2 nvarchar(30) NOT NULL;

SQL tested successfully , the alter table XXX alter column is re-entering.

To have a create table command with a re-entering feature =>
IF NOT EXISTS (SELECT * FROM sys.objects
WHERE object_id = OBJECT_ID(N'[dbo].[TEST_SEB]') AND type in (N'U'))
BEGIN
create TABLE TEST_SEB (
COL_1 nvarchar(10),
COL_2 nvarchar(10) NOT NULL
)
END;