Select constraint_name, search_condition  
From USER_CONSTRAINTS
Where table_name = 'your table name';

Alter Table your_table Drop Constraint constraint_name;
constraint_name是上一条语句查出来的

--非空
Alter Table your_table Modify field_name NOT NULL;

--主键
Alter Table your_table Add Constraint tb_pk Primary Key( field_name );

--外键
Alter Table your_table Add Constraint tb_fk Foreign Key( field_name ) 
References your_table2(field_name);

修改字段的默认值
Alter Table tbl_name modify col_name default(col_value);