MySQL数据库之视图

视图

• 概述

  • 视图是一张虚拟表，它表示一张表的部分数据或多张表的综合数据，其结构和数据是建立在对表的查询基础上
  • 视图中并不存放数据，而是存放在视图所引用的原始表（基表）中
  • 同一张原始表，根据不同用户的不同需求，可以创建不同的视图

• 作用

  • 筛选表中的行
  • 防止未经许可的用户访问敏感数据
  • 隐藏数据表的结构
  • 降低数据表的复杂程度

创建视图

• 语法

-- 创建视图
create view 视图名
as 
	select 语句；
	
-- 查询视图
select 列名 from 视图

MariaDB [sel]> create view best
    -> as
    -> select * from grades;
# `Query OK, 0 rows affected (0.012 sec)`

MariaDB [sel]> select math from best;
+------+
| math |
+------+
|   96 |
|   91 |
|   94 |
|   94 |
+------+
# `4 rows in set (0.008 sec)`

• 视图可以使得降低SQL语句的复杂度

mysql> create view view2
    -> as
    -> select stuno,stusex,writtenexam,labexam from stuinfo natural join stumarks;
# `Query OK, 0 rows affected (0.01 sec)`

修改视图

• 语法

alter view 视图名
as
	select 语句

mysql> alter view view2
    -> as
    -> select stuname from stuinfo;
# `Query OK, 0 rows affected (0.00 sec)`

删除视图

• 语法

drop view [if exists ] 视图1，视图,...

mysql> drop view view2;
# `Query OK, 0 rows affected (0.00 sec)`

查看视图信息

• 方法一
  • 显示所有的表和视图

mysql> show tables;	

• 方法二
  • 精确查找视图（视图信息存储在information_schema下的views表中）

mysql> select table_name from information_schema.views;
+------------+
| table_name |
+------------+
| view1      |
+------------+
# `1 row in set (0.05 sec)`

• 方法三
  • 通过表的comment属性查询视图

-- 查询所有表和视图的详细状态信息
mysql> show table statusG;		
-- 只查找视图信息
mysql> show table status where comment='view'G   

查询视图的结构

mysql> desc view1;

查询创建视图的语法

mysql> show create view view1G;

视图算法

• 一般场景
  • 找出数学成绩最高的男生和女生

MariaDB [sel]> select * from (select * from grades order by math desc) tab group by sex;
+-------+------+---------+------+
| name  | sex  | chinese | math |
+-------+------+---------+------+
| Sunny | boy  |      93 |   96 |
| Marry | girl |      95 |   94 |
+-------+------+---------+------+
# `2 rows in set (0.001 sec)`

MariaDB [sel]> create view bestMath
    -> as
    -> select * from grades order by math desc;
# `Query OK, 0 rows affected (0.010 sec)`

MariaDB [sel]> select * from bestMath group by sex;
+-------+------+---------+------+
| name  | sex  | chinese | math |
+-------+------+---------+------+
| Sunny | boy  |      93 |   96 |
| Marry | girl |      95 |   94 |
+-------+------+---------+------+
# `2 rows in set (0.001 sec)`

视图的算法

• 视图的算法有

  • merge 合并算法
    • 将视图语句和外层语句合并后再执行
  • temptable 临时表算法
    • 将视图作为一个临时表来执行
  • undefined 未定义算法
    • 用哪种算法有MySQL决定，这是默认算法，视图一般会选merge算法
    • 重新通过视图实现

• 场景

  • 找出语文成绩最高的男生和女生

• 方法一

mysql> select * from (select * from stu order by ch desc) t group by stusex;
+--------+----------+--------+--------+---------+------------+------+------+
| stuNo  | stuName  | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+----------+--------+--------+---------+------------+------+------+
| s25321 | Tabm     | 女      |    23 |       9 | 河北       |   88 |   77 |
| s25318 | 争青小子 | 男      |     26 |      6 | 天津        |   86 |   92 |
+--------+----------+--------+--------+---------+------------+------+------+
# `2 rows in set (0.00 sec)`

• 方法二

mysql> create view view3
    -> as
    -> select * from stu order by ch desc;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from view3 group by stusex;
+--------+---------+--------+--------+---------+------------+------+------+
| stuNo  | stuName | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+---------+--------+--------+---------+------------+------+------+
| s25301 | 张秋丽   | 男     |     18 |      1 | 北京        |   80 | NULL |
| s25303 | 李斯文   | 女     |     22 |      2 | 北京        |   55 |   82 |
+--------+---------+--------+--------+---------+------------+------+------+
# `2 rows in set (0.00 sec)`

• 方法三

mysql> create or replace algorithm=temptable view view3
    -> as
    -> select * from stu order by ch desc;
# `Query OK, 0 rows affected (0.00 sec)`

mysql> select * from view3 group by stusex;
+--------+----------+--------+--------+---------+------------+------+------+
| stuNo  | stuName  | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+----------+--------+--------+---------+------------+------+------+
| s25321 | Tabm     | 女     |     23 |       9 | 河北       |   88 |   77 |
| s25318 | 争青小子 | 男      |     26 |       6 | 天津       |   86 |   92 |
+--------+----------+--------+--------+---------+------------+------+------+
# `2 rows in set (0.00 sec)`

• 结论
  • 方法一和方法二的结果不一样，这是因为视图的算法造成的
  • 方法三指定算法为临时表算法，和子查询结果一致