View of MySQL introductory learning (III)

1. View
   Concept ： A virtual table extracted from one or more tables , To manipulate virtual tables CRUD Exactly the same as the original table

advantage ：
1. simple , After extracting from multiple tables , Just manage the virtual tables
2. Security , All you can see is the fields extracted from the virtual table
3. Logical independence

shortcoming ：
1. There's no performance improvement , Simple tables can become complex
2. It's inconvenient to modify , In many cases, you cannot modify the virtual table

Create view ：

create view myView as
select employee_id,first_name,last_name,salary from t_employees;

select * from myView;
update myView set salary=30000 where employee_id=100;

Modify the view ：
The way 1： Create or modify

CREATE OR REPLACE VIEW myView AS
select employee_id,first_name,last_name from t_employees;

The way 2： Modify the view

alter VIEW myView AS
select employee_id,first_name,last_name,salary from t_employees;

Delete view

drop view myView;

View immutability ：

create view myView as
select department_id,count(*) from t_employees group by department_id;

update myView set department_id=119 where department_id=10;

2. Comprehensive practice
Import data first , Then complete the following exercise ：

1. Query orders of all users

 select * from user u inner JOIN orders o on u.userid=o.userid;

2. Query the user id by 1 All order details for

 select * from user u INNER JOIN orders o on u.userid=o.userid
                      INNER JOIN orderitem oi on o.oid= oi.oid where u.userid=1;

3. View the order of Zhang San

 select * from orders where userid=(select userid from user where username=' Zhang San ');

4. The price of the queried order is greater than 800 All user information for .

select * from user where userid in (select DISTINCT userid from orders where totalprice>800);

5. Query all order information , Each page shows 5 Data

 select * from orders limit 0,5;    first page 
 
 select * from orders limit 5,5;    The second page 

 select * from orders limit 10,5;   The third page 

law ：
The current page is known , Page size ( Number of entries per page ), Find starting subscript
Start subscript = ( The current page -1)* Page size