ALTER TABLE <table name> ALTER <field name> [NOT] NULL
ALTER DOMAIN <domain name> [NOT] NULL

ALTER TABLE tablename
<operation> [, <operation>];
<operation> ::= {
ADD <col_def>
| ADD <tconstraint>
| DROP colname
| DROP CONSTRAINT constr_name
| ALTER [COLUMN] colname {
TO new_colname
| TYPE <datatype>
| POSITION new_col_position
| SET DEFAULT {literal | NULL | <context_var>}
| DROP DEFAULT
|  SET NOT NULL 
|  DROP NOT NULL 
}
| ALTER [COLUMN] gencolname [TYPE <datatype>]
{GENERATED ALWAYS AS | COMPUTED [BY]} (<col_expr>)
| ALTER [COLUMN] idencolname RESTART [ WITH startvalue ]
}