dbms數(shù)據(jù)庫(kù)管理系統(tǒng)

DBMS College professor once realized that students feel sad when they see their friend's marks higher than them and it creates a negative impact on them. It gave the Professor an idea to create a view table in his student academic result database.

DBMS學(xué)院的教授曾經(jīng)意識(shí)到，當(dāng)學(xué)生看到朋友的標(biāo)記比他們高時(shí)會(huì)感到難過(guò)，這會(huì)對(duì)他們產(chǎn)生負(fù)面影響。 這給了教授一個(gè)想法， 可以在他的學(xué)生學(xué)術(shù)成績(jī)數(shù)據(jù)庫(kù)中創(chuàng)建視圖表 。

In the database, View is a virtual table that combines the result set of a stored query. It is very important when we want to restrict a certain user from accessing the entire database. View is dynamic and can be computed from the data in the database. Changing the data in a table alters the data shown in the view as well.

在數(shù)據(jù)庫(kù)中， View是一個(gè)虛擬表，它結(jié)合了存儲(chǔ)查詢的結(jié)果集 。 當(dāng)我們想限制某個(gè)用戶訪問(wèn)整個(gè)數(shù)據(jù)庫(kù)時(shí)，這一點(diǎn)非常重要。 視圖是動(dòng)態(tài)的，可以根據(jù)數(shù)據(jù)庫(kù)中的數(shù)據(jù)進(jìn)行計(jì)算。 更改表中的數(shù)據(jù)也會(huì)更改視圖中顯示的數(shù)據(jù)。

When the Professor applies this technique, the student got to see their marks only and thus create a positive impact on the students as they are now competing with the one person only, themselves.

當(dāng)教授應(yīng)用此技術(shù)時(shí)，學(xué)生只能看到自己的標(biāo)記，從而對(duì)學(xué)生產(chǎn)生積極的影響，因?yàn)樗麄儸F(xiàn)在僅與一個(gè)人自己競(jìng)爭(zhēng)。

In a relational database, a view is not the part of a relational schema.

在關(guān)系數(shù)據(jù)庫(kù)中， 視圖不是關(guān)系模式的一部分。

1.創(chuàng)建視圖 (1. Create view)

Syntax to create a view:

創(chuàng)建視圖的語(yǔ)法：

create or replaceview view_nameasselect column_name1, column_name2,...from table_namewhere condition;

Example:

例：

Suppose, we have to create a student view table of view10.

假設(shè)我們必須創(chuàng)建一個(gè)view10的學(xué)生視圖表。

create view view10 select marks from student where rollno = 10;

2.放下視圖 (2. Drop View)

Syntax to drop a view:

刪除視圖的語(yǔ)法：

drop view viewname;

Example:

例：

If view10 table has to be dropped, the command looks like:

如果必須刪除view10表，則命令如下所示：

drop view view10;

DBMS中視圖的優(yōu)點(diǎn) (Advantages of a view in DBMS)

Views can subset the data in a table.

視圖可以將表中的數(shù)據(jù)子集化。

Views can join and simplify the tables in a virtual table.

視圖可以聯(lián)接并簡(jiǎn)化虛擬表中的表。

Views do not require additional storage.

視圖不需要額外的存儲(chǔ)。

Views can hide the complexity of the database and the data the user must hide that.

視圖可以隱藏?cái)?shù)據(jù)庫(kù)的復(fù)雜性以及用戶必須隱藏的數(shù)據(jù)。

Views can act as aggregated tables where aggregated data (sum, average, etc.) are calculated and presented as part of data.

視圖可以用作匯總表，在匯總表中計(jì)算匯總數(shù)據(jù)(總和，平均值等)并將其顯示為數(shù)據(jù)的一部分。

Views can provide additional security from unauthorized users and unauthorized access.

視圖可以為未經(jīng)授權(quán)的用戶和未經(jīng)授權(quán)的訪問(wèn)提供額外的安全性。

DBMS中視圖的缺點(diǎn) (Disadvantages of a view in DBMS)

Database view may be slow if it is approved from a view table that is generated from another view.

如果從另一個(gè)視圖生成的視圖表中批準(zhǔn)了數(shù)據(jù)庫(kù)視圖，則數(shù)據(jù)庫(kù)視圖可能會(huì)變慢。

翻譯自: https://www.includehelp.com/dbms/views-in-dbms.aspx

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