deptno int constraint emp_deptno_fk references dept(deptno))级联约束
deptno int constraint emp_deptno_fk references dept(deptno) on delete set null)或者on delete cascade//级联删除
ALTER TABLE test_phone_tab disable constraint test_phone_pk; //禁用约束
ALTER TABLE test_phone_tab enable constraint test_phone_pk; //启用约束
序列：
SQL> create sequence test_seq increment by 1 start with 1 maxvalue 1000 nocycle cache 20;
SQL> create table t1(x int primary key, y int);
SQL> insert into t1 values (test_seq.nextval, 11); 反复执行
SQL> select * from t1;
索引：
主键和唯一性约束自动创建索引：
SQL> select constraint_name, constraint_type from user_constraints where table_name='EMPLOYEES';
SQL> select index_name, index_type from user_indexes where table_name='EMPLOYEES';
SQL> set autot on
SQL> select last_name from employees where employee_id=100; 走索引
SQL> select email from employees; 走索引
SQL> select last_name from employees where salary=2100; 全表扫描
SQL> create index emp_salary_ix on employees(salary);
SQL> select last_name from employees where salary=2100; 走索引
SQL> set autot off