Sql知識點總結

1、數據庫對象：表(table) 視圖(view) 序列(sequence) 索引(index) 同義詞(synonym)

1. 視圖(view) : 存儲起來的 select 語句

   create view emp_vu5
   as
   select employee_id, last_name, salary
   from employees
   where department_id = 90;
   
   select * from emp_vu5;
   
   select * from employees;
   
   update emp_vu5
   set last_name = 'King'
   where employee_id = 100;

--複雜視圖:只能查詢，不能增刪改

create view emp_vu6
as
select department_id, avg(salary) avg_sal
from employees
group by department_id;

select * from emp_vu6;

update emp_vu6
set avg_sal = 10000
where department_id = 100;

1. 序列(sequence) ：用於生成一組有規律的數值。(一般爲主鍵設置值)

   create sequence emp_seq4
      start with 1
      increment by 1
      maxvalue 100000
      nocache
      cycle;
   
   select emp_seq4.currval from dual;
   select emp_seq4.nextval from dual;

--序列的問題：裂縫 1). 多個數據庫對象使用同一個序列。 2). rollback 3). 發生異常

insert into emp(id, name)
values(emp_seq4.nextval, '張三');

select * from emp;

1. 索引(index) ：用於提升查詢效率
   --自動建立：數據服務器會爲具備惟一約束(主鍵約束，惟一約束)的列自動建立索引

   create table emp2(
       id number(10) primary key,
       name varchar2(30)
   );

--手動建立:

create index emp2_name_idx
on emp2(name);

--建立聯合索引

create index emp2_nameId_idx
on emp2(id, name);

1. 同義詞(synonym)

   create synonym d for departments;
   select * from d;

2. 表(table)
   DML : 數據操縱語言

insert into ... values ...
delete from ... where ...
update ... set ... where ...

select ...組函數(MAX/MIN/AVG/SUM/COUNT)
from... (內鏈接 join...on... 左外鏈接：left join...on... 右外鏈接: right join...on... 滿外鏈接：full join...on...)
where... between...and.../in(..., ..., ...)/like/is (not) null
group by 出如今 select 子句中的非分組函數，必定出如今 group by 子句後
having 過濾組函數
order by ... asc(升序)-默認 desc(降序)

DDL : 數據定義語言(create table/alter table/drop table/rename...to.../truncate table) DCL : 數據控制語言(commit; rollback; grant...to.../ revoke)