Mysql数据库（六）视图

　　一、视图是由数据库中的一个表或多个表导出的虚拟表。其作用是方便用户对数据的操作。

　　1.视图的概念

　　2.视图的作用

　　二、创建视图

　　创建视图是指在已经存在的数据表上建立视图。视图可以建立在一张表中，也可以建立在多张表中。

　　1.查看创建视图的权限

mysql> SELECT Select_priv,Create_view_priv FROM mysql.user WHERE user='root';
+-------------+------------------+
| Select_priv | Create_view_priv |
+-------------+------------------+
| Y           | Y                |
+-------------+------------------+
1 row in set (0.01 sec)

　　结果中两列的值都为Y，这表明root用户具有Select（查看）和Create_view（创建视图）的权限。

　　2.创建视图

mysql> SELECT * FROM tb_bookinfo;
+----------+-----------+--------+-----------+------+-------+------+----------+------------+------+----+
| barcode  | bookname  | typeid | author    | ISBN | price | page | bookcase | inTime     | del  | id |
+----------+-----------+--------+-----------+------+-------+------+----------+------------+------+----+
| 17120107 | Java King |      3 | LianJiang | 115  | 49.80 |  350 |        1 | 2018-04-17 |    0 |  1 |
| 17120108 | Lian      |      1 | QiaoJiang | 116  | 50.00 |  351 |        2 | 2018-04-18 |    0 |  2 |
| 17120109 | Tian King |      2 | TianJiang | 117  | 51.10 |  352 |        3 | 2018-04-19 |    0 |  3 |
+----------+-----------+--------+-----------+------+-------+------+----------+------------+------+----+
3 rows in set (0.00 sec)
 
mysql> SELECT * FROM tb_borrow;
+----+----------+--------+------------+------------+----------+--------+
| id | readerid | bookid | borrowTime | backTime   | operator | ifback |
+----+----------+--------+------------+------------+----------+--------+
|  1 |        1 |      1 | 2018-04-17 | 2018-04-20 | mr       |      1 |
|  2 |        1 |      2 | 2018-04-16 | 2018-04-21 | mr       |      0 |
+----+----------+--------+------------+------------+----------+--------+
2 rows in set (0.00 sec)
 
mysql> CREATE VIEW
    -> v_book(barcode,bookname,author,price,bookcase,borrowTime,backTime)
    -> AS SELECT barcode,bookname,author,price,bookcase,borrowTime,backTime
    -> FROM tb_bookinfo AS b,tb_borrow AS t WHERE b.typeid=t.id;
Query OK, 0 rows affected (0.53 sec)
 
mysql> SELECT * FROM v_book;
+----------+-----------+-----------+-------+----------+------------+------------+
| barcode  | bookname  | author    | price | bookcase | borrowTime | backTime   |
+----------+-----------+-----------+-------+----------+------------+------------+
| 17120108 | Lian      | QiaoJiang | 50.00 |        2 | 2018-04-17 | 2018-04-20 |
| 17120109 | Tian King | TianJiang | 51.10 |        3 | 2018-04-16 | 2018-04-21 |
+----------+-----------+-----------+-------+----------+------------+------------+
2 rows in set (0.00 sec)

　　3.创建视图的注意事项

　　三、视图操作

　　1.查看视图

　　（1）DESCRIBE语句

mysql> DESC v_book;
+------------+------------------+------+-----+---------+-------+
| Field      | Type             | Null | Key | Default | Extra |
+------------+------------------+------+-----+---------+-------+
| barcode    | varchar(30)      | YES  |     | NULL    |       |
| bookname   | varchar(70)      | YES  |     | NULL    |       |
| author     | varchar(30)      | YES  |     | NULL    |       |
| price      | float(8,2)       | YES  |     | NULL    |       |
| bookcase   | int(10) unsigned | YES  |     | NULL    |       |
| borrowTime | date             | YES  |     | NULL    |       |
| backTime   | date             | YES  |     | NULL    |       |
+------------+------------------+------+-----+---------+-------+
7 rows in set (0.00 sec)

　　（2）SHOW TABLE STATUS语句查看视图的信息，这里说明视图为虚拟表，与普通表是有区别的。

mysql> SHOW TABLE STATUS LIKE 'v_book'\G
*************************** 1. row ***************************
           Name: v_book
         Engine: NULL
        Version: NULL
     Row_format: NULL
           Rows: NULL
 Avg_row_length: NULL
    Data_length: NULL
Max_data_length: NULL
   Index_length: NULL
      Data_free: NULL
 Auto_increment: NULL
    Create_time: NULL
    Update_time: NULL
     Check_time: NULL
      Collation: NULL
       Checksum: NULL
 Create_options: NULL
        Comment: VIEW
1 row in set (0.00 sec)
 
mysql> SHOW TABLE STATUS LIKE 'tb_bookinfo'\G
*************************** 1. row ***************************
           Name: tb_bookinfo
         Engine: InnoDB
        Version: 10
     Row_format: Dynamic
           Rows: 3
 Avg_row_length: 5461
    Data_length: 16384
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2018-04-17 15:17:13
    Update_time: 2018-04-17 16:51:30
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options:
        Comment:
1 row in set (0.00 sec)

　　（3）SHOW CREATE VIEW语句查看视图的详细定义

mysql> SHOW CREATE VIEW v_book\G
*************************** 1. row ***************************
                View: v_book
         Create View: CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `v_book` AS select `b`.`barcode` AS `barcode`,`b`.`bookname` AS `bookname`,`b`.`author` AS `author`,`b`.`price` AS `price`,`b`.`bookcase` AS `bookcase`,`t`.`borrowTime` AS `borrowTime`,`t`.`backTime` AS `backTime` from (`tb_bookinfo` `b` join `tb_borrow` `t`) where (`b`.`typeid` = `t`.`id`)
character_set_client: gbk
collation_connection: gbk_chinese_ci
1 row in set (0.00 sec)

　　2.修改视图

　　修改视图是指修改数据库中已存在的表的定义。当基本表的某些字段发生变化时，可以通过修改视图来保持视图和基本表之间一致。

　　（1）CREATE OR REPLACE VIEW语句在视图已经存在的情况下，对视图进行修改；视图不存在时，可以创建视图。

mysql> CREATE OR REPLACE VIEW
    -> v_book(barcode,bookname,borrowTime,backTime)
    -> AS SELECT barcode,bookname,borrowTime,backTime
    -> FROM tb_bookinfo AS b,tb_borrow AS t WHERE b.typeid=t.id;
Query OK, 0 rows affected (0.01 sec)
 
mysql> DESC v_book;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| barcode    | varchar(30) | YES  |     | NULL    |       |
| bookname   | varchar(70) | YES  |     | NULL    |       |
| borrowTime | date        | YES  |     | NULL    |       |
| backTime   | date        | YES  |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

　　（2）ALTER VIEW语句改变了视图的定义，包括被索引视图，但不影响所依赖的存储过程或触发器。

mysql> ALTER VIEW
    -> v_book(barcode,borrowTime,backTime)
    -> AS SELECT barcode,borrowTime,backTime
    -> FROM tb_bookinfo AS b,tb_borrow AS t WHERE b.typeid=t.id
    -> WITH CHECK OPTION;
Query OK, 0 rows affected (0.00 sec)
 
mysql> DESC v_book;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| barcode    | varchar(30) | YES  |     | NULL    |       |
| borrowTime | date        | YES  |     | NULL    |       |
| backTime   | date        | YES  |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

　　3.更新视图

　　对视图的更新其实就是对表的更新，更新视图是通过视图来插入、更新和删除表中的数据。因为视图是一个虚拟表，其中没有数据，通过视图更新时，都是转换到基本表来更新。更新视图时，只能更新权限范围内的数据，超出了范围，就不能更新。

　　（1）更新视图，同时也把原来的表更新了

mysql> SELECT * FROM v_book;
+----------+------------+------------+
| barcode  | borrowTime | backTime   |
+----------+------------+------------+
| 17120108 | 2018-04-17 | 2018-04-20 |
| 17120109 | 2018-04-16 | 2018-04-21 |
+----------+------------+------------+
2 rows in set (0.00 sec)
 
mysql> UPDATE v_book SET borrowTime='2018-04-19' WHERE barcode='17120108';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0
 
mysql> SELECT * FROM v_book;
+----------+------------+------------+
| barcode  | borrowTime | backTime   |
+----------+------------+------------+
| 17120108 | 2018-04-19 | 2018-04-20 |
| 17120109 | 2018-04-16 | 2018-04-21 |
+----------+------------+------------+
2 rows in set (0.00 sec)
 
mysql> SELECT * FROM tb_borrow;
+----+----------+--------+------------+------------+----------+--------+
| id | readerid | bookid | borrowTime | backTime   | operator | ifback |
+----+----------+--------+------------+------------+----------+--------+
|  1 |        1 |      1 | 2018-04-19 | 2018-04-20 | mr       |      1 |
|  2 |        1 |      2 | 2018-04-16 | 2018-04-21 | mr       |      0 |
+----+----------+--------+------------+------------+----------+--------+
2 rows in set (0.00 sec)

　　（2）更新视图的限制

　　　　a.视图中包含COUNT()、SUM()、MAX()、MIN()等函数

　　　　b.视图中包含UNION、UNION ALL、DISTINCT、GROUP BY和HAVIG等关键字

　　　　c.常量视图

　　　　d.视图中的SELECT中包含子查询

　　　　f.由不可更新的视图导出的视图

　　　　g.创建视图时，ALGORITHM为TEMPTABLE类型

　　　　h.视图对应的表上存在没有默认值的列，而且该列没有包含在视图里。

　　4.删除视图

mysql> DROP VIEW IF EXISTS v_book;
Query OK, 0 rows affected (0.00 sec)
 
mysql> SELECT * FROM v_book;
ERROR 1146 (42S02): Table 'db_library.v_book' doesn't exist