VIEW
Views are stored queries. A view acts as a
virtual table. A view consists of rows & columns just like table. the
difference between table and view is that view are definitions built on top of
other tables (or view).
There are some points to define a View as follows :
- It is used to restrict access to the database.
- Hide data compexity.
- A view is stored as a select statement in the database.
- DML operations on a view like Insert, Update, Delete affects.
There are several rules which SELECT statement
has to follows:
- In the SELECT statement Subquery cannot be included.
-
Variables such as local, user and session variables cannot be used in the SELECT statement.
- Prepared statement cannot be used in the view.
-
Temporary tables or views cannot be used in the SELECT statements and any tables or views which referred by views must exists.
-
View cannot be associated with triggers.
How to create a View
Syntax :
CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
We can use the show create View command in the
following example :
For Example :
CREATE TABLE Employee(
id int,
first_name VARCHAR(15),
last_name VARCHAR(15),
start_date DATE,
end_date DATE,
salary FLOAT(8,2),
city VARCHAR(10),
description VARCHAR(15)
);
Insert the values in the table :
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City,
-> Description) values (1,'Jason', 'Martin', '19960725', '20060725', 1234.56, 'Toronto', 'Programmer');
Query OK, 1 row affected (0.00 sec)
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City,
-> Description) values(2,'Alison', 'Mathews', '19760321', '19860221', 6661.78, 'Vancouver','Tester');
Query OK, 1 row affected (0.00 sec)
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City,
-> Description) values(3,'James', 'Smith', '19781212', '19900315', 6544.78, 'Vancouver','Tester');
Query OK, 1 row affected (0.00 sec)
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City,
-> Description) values(4,'Celia', 'Rice', '19821024', '19990421', 2344.78, 'Vancouver','Manager');
Query OK, 1 row affected (0.02 sec)
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City,
-> values(5,'Robert', 'Black', '19840115', '19980808', 2334.78, 'Vancouver','Tester');
Query OK, 1 row affected (0.00 sec)
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City,
-> Description) values(6,'Linda', 'Green', '19870730', '19960104', 4322.78,'New York', 'Tester');
Query OK, 1 row affected (0.00 sec)
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City,
-> Description) values(7,'David', 'Larry', '19901231', '19980212', 7897.78,'New York', 'Manager');
Query OK, 1 row affected (0.00 sec)
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City,
-> Description) values(8,'James', 'Cat', '19960917', '20020415', 1232.78,'Vancouver', 'Tester');
Query OK, 1 row affected (0.00 sec)
select * from Employee;
+------+------------+-----------+------------+------------+---------+-----------+-------------+
| id | first_name | last_name | start_date | end_date | salary | city | description |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
| 1 | Jason | Martin | 1996-07-25 | 2006-07-25 | 1234.56 | Toronto | Programmer |
| 2 | Alison | Mathews | 1976-03-21 | 1986-02-21 | 6661.78 | Vancouver | Tester |
| 3 | James | Smith | 1978-12-12 | 1990-03-15 | 6544.78 | Vancouver | Tester |
| 4 | Celia | Rice | 1982-10-24 | 1999-04-21 | 2344.78 | Vancouver | Manager |
| 5 | Robert | Black | 1984-01-15 | 1998-08-08 | 2334.78 | Vancouver | Tester |
| 6 | Linda | Green | 1987-07-30 | 1996-01-04 | 4322.78 | New York | Tester |
| 7 | David | Larry | 1990-12-31 | 1998-02-12 | 7897.78 | New York | Manager |
| 8 | James | Cat | 1996-09-17 | 2002-04-15 | 1232.78 | Vancouver | Tester |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
Create a view in this command :
CREATE OR REPLACE VIEW myView AS
SELECT id, first_name, city FROM employee
WHERE id = 3 WITH LOCAL CHECK OPTION;
View table:- SELECT * FROM myView;
![create-view.gif]()
mysql> CREATE OR REPLACE VIEW myView1 AS
-> SELECT id, first_name FROM myView;
Query OK, 0 rows affected (0.01 sec)
View table:- select * from myView1;
![replace.gif]()
mysql> SHOW CREATE VIEW myView\G
![show.gif]()
How to drop a View :
mysql> drop view myView1;
Query OK, 0 rows affected (0.00 sec)