Answers for "alter column to not null"

Change A filed to not null

ALTER TABLE
  clients
ALTER COLUMN
  phone
    NVARCHAR(20) NOT NULL;

SQL Alter column to be not null

ALTER TABLE table_name
ALTER COLUMN col_name data_type NOT NULL;

change sql column from null to not null

ALTER TABLE [TABLE_NAME] CHANGE [COLUMN_NAME] [COLUMN_NAME] [COLUMN_TYPE] NOT NULL DEFAULT [DEFAULT];