all_constraints
表字段Constraint_type值含義
| Type Code | Type Description | Acts On Level |
| C | Check on a table | Column |
| O | Read Only on a view | Object |
| P | Primary Key | Object |
| R | Referential AKA Foreign Key | Column |
| U | Unique Key | Column |
| V | Check Option on a view | Object |
例：ORACLE刪除非空約束
1.查出約束名：
select constraint_name, search_condition
from user_constraints
where table_name = '表名（大寫）'
and constraint_type = 'C';
2.alter table 表名 drop constraint 約束名;
創建表的同時創建主鍵約束
（1）無命名
create table student (
studentid int primary key not null,
studentname varchar(8),
age int);
（2）有命名
create table students (
studentid int ,
studentname varchar(8),
age int,
constraint yy primary key(studentid));
2、刪除表中已有的主鍵約束
（1）無命名
可用 select * from user_cons_columns;
或 select constraint_name, search_condition
from user_constraints
where table_name = '表名（大寫）'
and constraint_type = 'P';
查找表中主鍵名稱得student表中的主鍵名爲SYS_C002715
alter table student drop constraint SYS_C002715;
（2）有命名
alter table students drop constraint yy;
3、向表中添加主鍵約束
alter table student add constraint pk_student primary key(studentid);
