This document discusses database views in MySQL. It defines a view as a virtual table composed of the result set of a SELECT query. Views allow users to retrieve and update data as if it were a table. The document outlines how to create views from SELECT statements, how to make views updatable by following certain rules, and how to manage views using SHOW, ALTER, and DROP commands.

1. Hệ quản trị cơ sở dữ liệu
Khung nhìn (View)
Dư Phương Hạnh
Bộ môn Hệ thống thông tin
Khoa CNTT, trường Đại học Công nghệ
Đại học Quốc gia Hanoi
hanhdp@vnu.edu.vn

2. Outline
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Introduction
Create View
Updatable View
Manage View
Tài liệu tham khảo:
http://www.mysqltutorial.org/mysql-views-tutorial.a
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3. Introduction
 By definition, a database view is a virtual or logical
table which is composed of result set of a SELECT
query.
 You can retrieve and update data on it in the same
way with table.
 View is dynamic because it is not related to the
physical schema. It is stored as view definition as
SELECT statements.
 When the tables which are the source data of a view
changes; the data in the view change also.
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4. Introduction

Two kinds:
1. Virtual = not stored in the database; just a query for
constructing the relation.
2. Materialized = actually constructed and stored.
– Problem: each time a base table changes, the
materialized view may change.
• Cannot afford to recompute the view with each change.
–
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Solution: Periodic reconstruction of the materialized
view, which is otherwise “out of date.”
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5. Giới thiệu
 Advantages:
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–
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Simplify complex query
Limited access data to the specific users
Provide extra security
Computed column
Backward compatibility
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6. MySQL View
 MySQL supports views from version 5.x and almost
views’ features conform to the SQL: 2003 standard.
– MySQL creates a temporary table based on the query
which defined the view and then execute the input query
on this table.
– MySQL first combines the input query and query which
defined the view, then MySQL executes this query.
 MySQL also allows you to create views of views.
 If the cached is enabled, the query against view is
stored in the cache  increases the performance of
query by pulling data from the buffering instead of
querying data from physical hard disk.
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7. Create View
CREATE VIEW [database name].[view name]
AS
[SELECT statement]
In SELECT statement:
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you can query any tables or views existed in the db.
Subquery cannot be included
Any variables cannot be used
The prepared statement cannot be used in the view.
Temporary tables or views cannot be used; any tables or
views which referred by views must exists.
– View cannot be associated with triggers.
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8. Examples
CREATE VIEW SalePerOrder
AS
SELECT orderNumber,
SUM (quantityOrdered * priceEach) total
FROM orderDetails
GROUP BY orderNumber
ORDER BY total DESC
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9. Examples
CREATE VIEW vwProducts
AS
SELECT productCode, productName, buyPrice
FROM products
WHERE buyPrice > (
SELECT AVG (buyPrice)
FROM products)
ORDER BY buyPrice DESC
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10. Updatable View
 The SELECT statement which defines View has to follow
several following rules:
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SELECT statement must not reference to more than one table.
SELECT statement must not use GROUP BY or HAVING clause.
SELECT statement must not use DISTINCT in the selection list.
SELECT statement must not reference to the view that is not
updateable
– SELECT statement must not contain any expression (aggregates,
functions, computed columns…)
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11. Updatable View
examples
CREATE VIEW officeInfo
AS
SELECT officeCode, phone, city
FROM offices
SELECT * FROM officeInfo
UPDATE officeInfo
SET phone = '+33 14 723 5555'
WHERE officeCode = 4
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12. Managing View
 Displaying a View
– SHOW CREATE VIEW <view_name>
 Modifying a View
– ALTER VIEW <view_name> AS <sql_statement>
 Removing a View
– DROP VIEW [IF EXISTS] <view_name>
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