Oracle alter table instance

-- In this case, you need to add several constraints to the table. The syntax for adding constraints is as follows:
-- Alter table name add constraint name constraint type (constraint field)
-- Delete constraints:
-- Alter table book drop constraint book_pid_fk;

-- Pk add primary key
Alter table dirk_dept add constraint dept_pk primary key (id );
Alter table dirk_company add constraint comp_pk primary key (id );
Alter table dirk_emp add constraint emp_pk primary key (id );

-- Fk add foreign key
Alter table dirk_dept add constraint dept_fk_comp foreign key (companyid) references dirk_company;
Alter table dirk_emp add constraint emp_fk_dept foreign key (deptid) references dirk_dept; -- you can add on delte cascade
-- Foreign key reference behavior delete cascade/delete set null/(default) delete on action

-- Uk add constraint unique
Alter table dirk_emp add constraint dirk_emp_num_uk unique (num );

-- Ck add check
Alter table dirk_emp add constraint ck_dirk_emp_num check (num between 0 and 250)
Alter table dirk_emp add constraint

-- Add column
Alter table dirk_emp add (score number (20 ));

-- Add comment
Comment on column dirk_emp.score is 'employee score ';

-- Modify column property -- it will current error when exist null value
Alter table dirk_emp modify score not null;
-- Modify default value
Alter table dirk_emp modify score default 100;
-- Modify length or type
Alter table dirk_emp modify name number (20) -- varchar2 (50)

-- Remove column
Alter table dirk_emp drop column score;

-- Contraint disable or enable: enable Constraints
Alter table dirk_emp disable constraint dirk_emp_num_uk;
Alter table dirk_emp enable constraint dirk_emp_num_uk;