檢視

• 概述

  • 檢視是一張虛擬表，它表示一張表的部分資料或多張表的綜合資料，其結構和資料是建立在對錶的查詢基礎上
  • 檢視中並不存放資料，而是存放在檢視所引用的原始表（基表）中
  • 同一張原始表，根據不同使用者的不同需求，可以建立不同的檢視

• 作用

  • 篩選表中的行
  • 防止未經許可的使用者訪問敏感資料
  • 隱藏資料表的結構
  • 降低資料表的複雜程度

建立檢視

• 語法

-- 建立檢視
create view 檢視名
as 
	select 語句；
	
-- 查詢檢視
select 列名 from 檢視

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)`
 

• 檢視可以使得降低SQL語句的複雜度

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

修改檢視

• 語法

alter view 檢視名
as
	select 語句

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

刪除檢視

• 語法

drop view [if exists ] 檢視1，檢視,...

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

檢視檢視資訊

• 方法一
  • 顯示所有的表和檢視

mysql> show tables;	

• 方法二
  • 精確查詢檢視（檢視資訊儲存在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
  • 通過表的comment屬性查詢檢視

-- 查詢所有表和檢視的詳細狀態資訊
mysql> show table status\G;		
-- 只查詢檢視資訊
mysql> show table status where comment='view'\G   

查詢檢視的結構

mysql> desc view1;

查詢建立檢視的語法

mysql> show create view view1\G;

檢視演算法

• 一般場景
  • 找出數學成績最高的男生和女生

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)`

檢視的演算法

• 檢視的演算法有

  • merge 合併演算法
    • 將檢視語句和外層語句合併後再執行
  • temptable 臨時表演算法
    • 將檢視作為一個臨時表來執行
  • undefined 未定義演算法
    • 用哪種演算法有MySQL決定，這是預設演算法，檢視一般會選merge演算法
    • 重新通過檢視實現

• 場景

  • 找出語文成績最高的男生和女生

• 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 | 爭青小子 | 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 | 張秋麗   | 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 | 爭青小子 | 男      |     26 |       6 | 天津       |   86 |   92 |
+--------+----------+--------+--------+---------+------------+------+------+
# `2 rows in set (0.00 sec)`

• 結論
  • 方法一和方法二的結果不一樣，這是因為檢視的演算法造成的
  • 方法三指定演算法為臨時表演算法，和子查詢結果一致