110视图

#1. 使用视图以后就无需每次都重写子查询的sql，但是这么效率并不高，还不如我们写子查询的效率高

#2. 而且有一个致命的问题：视图是存放到数据库里的，如果我们程序中的sql过分依赖于数据库中存放的视图，那么意味着，一旦sql需要修改且涉及到视图的部分，则必须去数据库中进行修改，而通常在公司中数据库有专门的DBA负责，你要想完成修改，必须付出大量的沟通成本DBA可能才会帮你完成修改，极其地不方便

# =============================创建视图

select * from emp inner join dep on emp.dep_id = dep.id;


create view emp2dep as select emp.*,dep.name as dep_name from emp inner join dep on emp.dep_id = dep.id;


mysql> update emp2dep set name="EGON" where id=1;
Query OK, 1 row affected (0.05 sec)
Rows matched: 1  Changed: 1  Warnings: 0
mysql> select * from emp2dep;
+----+-----------+--------+------+--------+--------------+
| id | name      | sex    | age  | dep_id | dep_name     |
+----+-----------+--------+------+--------+--------------+
|  1 | EGON      | male   |   18 |    200 | 技术         |
|  2 | alex      | female |   48 |    201 | 人力资源     |
|  3 | wupeiqi   | male   |   38 |    201 | 人力资源     |
|  4 | yuanhao   | female |   28 |    202 | 销售         |
|  5 | liwenzhou | male   |   18 |    200 | 技术         |
+----+-----------+--------+------+--------+--------------+
5 rows in set (0.00 sec)

mysql>
mysql>
mysql> select * from emp;
+----+------------+--------+------+--------+
| id | name       | sex    | age  | dep_id |
+----+------------+--------+------+--------+
|  1 | EGON       | male   |   18 |    200 |
|  2 | alex       | female |   48 |    201 |
|  3 | wupeiqi    | male   |   38 |    201 |
|  4 | yuanhao    | female |   28 |    202 |
|  5 | liwenzhou  | male   |   18 |    200 |
|  6 | jingliyang | female |   18 |    204 |
|  7 | lili       | female |   48 |   NULL |
+----+------------+--------+------+--------+
7 rows in set (0.00 sec)

mysql>

# =============================修改视图
alter view emp2dep as 查询语句;

# =============================删除视图
drop view emp2dep;

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