SQL 中简单视图和复杂视图的区别

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SQL 中的视图，作为来自一个或多个表的数据的逻辑子集。视图用于限制数据访问。视图不包含其自身的数据，但包含类似窗口，通过该窗口可以查看或更改表中的数据。视图所基于的表称为基表。

SQL 中有两种类型的视图：简单视图和复杂视图。简单视图只能包含一个基表。可以在多个基表上构建复杂视图。特别是，复杂视图可以包含：连接条件、group by 子句、order by 子句。

这些类型的视图之间的主要区别是：

Simple View	Complex View
Contains only one single base table or is created from only one table.	Contains more than one base tables or is created from more than one tables.
We cannot use group functions like MAX(), COUNT(), etc.	We can use group functions.
Does not contain groups of data.	It can contain groups of data.
DML operations could be performed through a simple view.	DML operations could not always be performed through a complex view.
INSERT, DELETE and UPDATE are directly possible on a simple view.	We cannot apply INSERT, DELETE and UPDATE on complex view directly.
Simple view does not contain group by, distinct, pseudocolumn like rownum, columns defiend by expressions.	It can contain group by, distinct, pseudocolumn like rownum, columns defiend by expressions.
Does not include NOT NULL columns from base tables.	NOT NULL columns that are not selected by simple view can be included in complex view.

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