MySQL学习--（视图、存储引擎、事务）

MySQL学习–（视图、存储引擎、事务）

视图

1).什么是视图？

view 又称虚拟表，view其实就一条查询SQL语句的结果集==>将常用的SQL查询结果集虚拟为一张表存放在内存中
create view as 视图名 (查询SQL语句结果集);–当再次使用时:select * from 视图名

2).视图有什么用？【视图实际上存储的就是SQL语句】

①权限的控制!比如：某几个列允许用户查询，而其他列不允许，可以通过视图开放其中的一部分列，达到权限的控制
②简化复杂的查询!比如：查询每个栏目下的商品的平均价格并按平均价格排序，然后查出平均价格前3高的栏目
①create view v as select cat_id, avg(shop_price) as pj from goods group by cat_id
②select * from v order by pj limit 0,3

3).视图能不能更新删除修改

①视图虚拟表： 是物理表的一个"投影",两者是相互影响的；更改物理表，虚拟表也会更改，同理，更改虚拟表，物理表也会更改！

但是：如果虚拟表中含有函数(经过计算…)，则不能修改!（即物理表和虚拟表的列能一一对应，则虚拟表中该列能修改改一行影响一行）

• create view as v select cat_id, avg(shop_price) as pj from goods group by cat_id
• update v set pj = 80 where cat_id=11;–报错！因为修改结果不能正确映射goods表中所有shop_price中

同理：增加和删除操作也是和修改一样

4).视图放在什么地方？

1.对于VIEW存储的SQL语句是简单的select语句，所以当对视图查询时候就是对SQL语句的拼接==>对物理表的间接拼接查询(合并：merge)

2.对于VIEW存储的SQL语句已经是逻辑复杂的select语句了，这时对视图的拼接查询会更麻烦！这时候mysql会先执行视图的创建语句，把结果集形成一张临时表，再对临时表(temptable)进行操作

MySQL数据库中可以通过algorithm(算法)定义对视图的处理情况 create algorithm = merge/temptalbe view v_name as …
[不写该属性，则由MySQL自行判断]

mysql> create view v1 as select * from goods where shop_price>300;
Query OK, 0 rows affected (0.38 sec)

mysql> select * from v1 where shop_price < 500;
+----------+---------------+--------+----------+-----------+--------------+------------+--------------+-------------+
| goods_id | goods_name    | cat_id | brand_id | goods_sn  | goods_number | shop_price | market_price | click_count |
+----------+---------------+--------+----------+-----------+--------------+------------+--------------+-------------+
|        8 | 飞利浦9@9v    |      3 |        4 | ecs000008 |            1 |     399.00 |       478.79 |          10 |
+----------+---------------+--------+----------+-----------+--------------+------------+--------------+-------------+
1 row in set (0.00 sec)

mysql> create algorithm=merge view v2
    -> as select * from goods where shop_price>300;
Query OK, 0 rows affected (0.23 sec)

mysql> select * from v2 where shop_price <500;
+----------+---------------+--------+----------+-----------+--------------+------------+--------------+-------------+
| goods_id | goods_name    | cat_id | brand_id | goods_sn  | goods_number | shop_price | market_price | click_count |
+----------+---------------+--------+----------+-----------+--------------+------------+--------------+-------------+
|        8 | 飞利浦9@9v    |      3 |        4 | ecs000008 |            1 |     399.00 |       478.79 |          10 |
+----------+---------------+--------+----------+-----------+--------------+------------+--------------+-------------+
1 row in set (0.00 sec)

mysql> create algorithm=temptable view v3 as select * from goods order by cat_id asc,shop_price desc;
Query OK, 0 rows affected (0.18 sec)

mysql> select * from goods order by cat_id asc,shop_price desc group by cat_id;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'group by cat_id' at line 1
mysql> select * from v3 group by cat_id;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'shop.v3.goods_id' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
mysql> select cat_id  from v3 group by cat_id;
+--------+
| cat_id |
+--------+
|      2 |
|      3 |
|      4 |
|      5 |
|      8 |
|     11 |
|     13 |
|     14 |
|     15 |
+--------+
9 rows in set (0.00 sec)

存储引擎

mysql> show engines \G
*************************** 1. row ***************************
      Engine: MEMORY
     Support: YES
     Comment: Hash based, stored in memory, useful for temporary tables
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 2. row ***************************
      Engine: MRG_MYISAM
     Support: YES
     Comment: Collection of identical MyISAM tables
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 3. row ***************************
      Engine: CSV
     Support: YES
     Comment: CSV storage engine
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 4. row ***************************
      Engine: FEDERATED
     Support: NO
     Comment: Federated MySQL storage engine
Transactions: NULL
          XA: NULL
  Savepoints: NULL
*************************** 5. row ***************************
      Engine: PERFORMANCE_SCHEMA
     Support: YES
     Comment: Performance Schema
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 6. row ***************************
      Engine: MyISAM
     Support: YES
     Comment: MyISAM storage engine
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 7. row ***************************
      Engine: InnoDB
     Support: DEFAULT
     Comment: Supports transactions, row-level locking, and foreign keys
Transactions: YES
          XA: YES
  Savepoints: YES
*************************** 8. row ***************************
      Engine: BLACKHOLE
     Support: YES
     Comment: /dev/null storage engine (anything you write to it disappears)
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 9. row ***************************
      Engine: ARCHIVE
     Support: YES
     Comment: Archive storage engine
Transactions: NO
          XA: NO
  Savepoints: NO
9 rows in set (0.00 sec)

事务

事务：一般用于账户金额相关的内容操作。

事务可以保证多个操作原子性，要么全成功，要么全失败。对于数据库来说事务保证批量的 DML 要么全成功，要么全
失败。事务具有四个特征 ACID

• a) 原子性（ Atomicity）： 整个事务中的所有操作，必须作为一个单元全部完成（或全部取消）。
• b) 一致性（ Consistency）： 在事务开始之前与结束之后，数据库都保持一致状态。
• c) 隔离性(Isolation)：一个事务不会影响其他事务的运行。
• d) 持久性(Durability)：在事务完成以后，该事务对数据库所作的更改将持久地保存在数据库之中，并不会被回滚。

事务基本概念：

• a) 事务（ Transaction）：一批操作（一组 DML）
• b) 开启事务（ Start Transaction）
• c) 回滚事务（ rollback）
• d) 提交事务（ commit）
• e) SET AUTOCOMMIT：禁用或启用事务的自动提交模式

当执行 DML 语句是其实就是开启一个事务
关于事务的回滚需要注意：只能回滚 insert、 delete 和 update 语句，不能回滚 select（回滚 select 没有任何意义），对于
create、 drop、 alter 这些无法回滚.

事务只对 DML 有效果。

rollback，或者 commit 后事务就结束了

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> update a2 set money = money+1000 where uname='张三';
Query OK, 1 row affected (0.04 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from a2;
+--------+---------+
| uname  | money   |
+--------+---------+
| 张三   | 7666.66 |
| 李四   | 7777.77 |
+--------+---------+
2 rows in set (0.00 sec)

mysql> updata a2 set money = money -1000 where uname='李四';
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'updata a2 set money = money -1000 where uname='李四'' at line 1
mysql> rollback;
Query OK, 0 rows affected (0.14 sec)

mysql> select * from a2;
+--------+---------+
| uname  | money   |
+--------+---------+
| 张三   | 6666.66 |
| 李四   | 7777.77 |
+--------+---------+
2 rows in set (0.00 sec)

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> update a2 set money = money+1000 where uname='张三';
Query OK, 1 row affected (0.10 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> update a2 set money = money -1000 where uname='李四';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> commit;
Query OK, 0 rows affected (0.17 sec)

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> update a2 set money = money +500 where uname='李四';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> update a2 set money = money-1000 where uname='张三';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> commit;
Query OK, 0 rows affected (0.12 sec)

mysql> select * from a2;
+--------+---------+
| uname  | money   |
+--------+---------+
| 张三   | 6666.66 |
| 李四   | 7277.77 |
+--------+---------+
2 rows in set (0.00 sec)