How to implement constraints in SQL

Editor to share with you how to achieve SQL constraints, I believe that most people do not know much about it, so share this article for your reference, I hope you can learn a lot after reading this article, let's go to understand it!

1. Five types

Non-empty not null unique unique primary key primary foreign key foreign check check

Description:

The foreign key on delete cascade/set null option is specified when the foreign key is defined

2. View

User_constraints

User_cons_colums

3. Create

Column level: constraint emp_id primary key is set along with the column

Table level: constraint con_pk primary key (emp_id) set after all column definitions are complete

4. Delete

Alter table emp drop constraint

Delete not null: alter table emp02 modify deptno null

Cascade options:

Alter table dept02 drop constraint pk_dept02_deptno cascade

When you delete a primary key constraint on a parent table, the cascade deletes the foreign key constraint of the child table first.

5. Add

Alter table emp add constraint

Add not null: alter table emp02 modify empno not null

6. Invalidation takes effect

Alter table.. Disable constraint...

Alter table.. Enable constraint...

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