Правильный синтаксис оператора ALTER TABLE:

ALTER TABLE table { [ ALTER COLUMN column_name     { new_data_type [ ( precision [ , scale ] ) ]         [ COLLATE  collation_name  ]         [ NULL | NOT NULL ]         | {ADD | DROP } ROWGUIDCOL }     ]     | ADD         { [  column_definition  ]         |  column_name AS computed_column_expression         } [ ,...n ]     | [ WITH CHECK | WITH NOCHECK ] ADD         {  table_constraint  } [ ,...n ]}

ALTER TABLE table { [ ALTER COLUMN column_name     | DROP         { [ CONSTRAINT ] constraint_name             | COLUMN column } [ ,...n ]     | { CHECK | NOCHECK } CONSTRAINT         { ALL | constraint_name [ ,...n ] }     | { ENABLE | DISABLE } TRIGGER         { ALL | trigger_name [ ,...n ] } }