注释：视图总是显示最近的数据。每当用户查询视图时，数据库引擎通过使用 SQL 语句来重建数据。
SQL CREATE VIEW 实例
可以从某个查询内部、某个存储过程内部，或者从另一个视图内部来使用视图。通过向视图添加函数、join 等等，我们可以向用户精确地提交我们希望提交的数据。

样本数据库 Northwind 拥有一些被默认安装的视图。视图 "Current Product List" 会从 Products 表列出所有正在使用的产品。这个视图使用下列 SQL 创建：

CREATE VIEW [Current Product List] AS
SELECT ProductID,ProductName
FROM Products
WHERE Discontinued=No我们可以查询上面这个视图：

SELECT * FROM [Current Product List]Northwind 样本数据库的另一个视图会选取 Products 表中所有单位价格高于平均单位价格的产品：

CREATE VIEW [Products Above Average Price] AS
SELECT ProductName,UnitPrice
FROM Products
WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)

我们可以像这样查询上面这个视图：

SELECT * FROM [Products Above Average Price]另一个来自 Northwind 数据库的视图实例会计算在 1997 年每个种类的销售总数。请注意，这个视图会从另一个名为 "Product Sales for 1997" 的视图那里选取数据：

CREATE VIEW [Category Sales For 1997] AS
SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales
FROM [Product Sales for 1997]
GROUP BY CategoryName

我们可以像这样查询上面这个视图：

SELECT * FROM [Category Sales For 1997]我们也可以向查询添加条件。现在，我们仅仅需要查看 "Beverages" 类的全部销量：

SELECT * FROM [Category Sales For 1997]
WHERE CategoryName='Beverages'

SQL 更新视图
您可以使用下面的语法来更新视图：

SQL CREATE OR REPLACE VIEW Syntax
CREATE OR REPLACE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition

现在，我们希望向 "Current Product List" 视图添加 "Category" 列。我们将通过下列 SQL 更新视图：

CREATE VIEW [Current Product List] AS
SELECT ProductID,ProductName,Category
FROM Products
WHERE Discontinued=No

SQL 撤销视图
您可以通过 DROP VIEW 命令来删除视图。

SQL DROP VIEW Syntax
DROP VIEW view_name
OR REPLACE VIEW ``

下面为老外上的教程

your_view_name>`

AS

...其次是正常的SQL的SELECT。这个SELECT可以包含一个WHERE子句或其他需要，可以对SELECT语句放在别的事情。该方案是无止境的。这实际上取决于视图的目的。

正如你可以看到在我们看来，我们正在格式化的姓氏和名字。这是一个很常见的事做有一个观点，即我们已经这样做了保存有写在每一个查询的WHERE这是一个要求的功能。你也可以看到，我们已经采取的出生日期列和计算年龄。

执行视图
执行一个SQL视图
下面的例子显示所有从视图代码。你也可以做一个SELECT*，或进一步列你想看到的。您还可以添加额外的行的看法，因为我们的做法。

SELECT FIRSTNAME ,
LASTNAME ,
BIRTH_DTTM ,
FULLNAME_FL ,
AGE
FROM VW_STUDENTS1
WHERE AGE IS NOT NULL

/

Creating a View containing one or more SQL Tables
Another key advantage of a view is that it allows us to join multiple tables together.

CREATE OR REPLACE VIEW vw_OCCUPIED_SEATS_BY_CLASS
AS
SELECT
c.COURSE_DESIGNATER_FK AS "COURSE",
b.SEAT_NUM ,
(a.FIRSTNAME || ' ' || a.LASTNAME) AS "STUDENT"

FROM STUDENTS a
JOIN CLASSREGISTRATION b
ON a.STUDENT_ID = b.STUDENT_ID_FK
JOIN CLASSES c
ON c.CLASSES_NUM = b.CLASSES_NUM
/

Above is a simple view that provides us with a listing of occupied/unoccupied seats for our classes. As you can see from the examples below, we can use this view in a variety of different ways. Note that for each scenario that we did not need to join any tables. The grunt work is already done.

Using our View
View a single class

SELECT COURSE ,
SEAT_NUM ,
STUDENT
FROM VW_OCCUPIED_SEATS_BY_CLASS
WHERE COURSE = 'Perl100' and STUDENT <> '1'
/

COURSE SEAT_NUM STUDENT
----------------------------------
Perl100 1 Madge Lowdown
Perl100 2 Robert Frapples
Perl100 3 Mary Lamacker
Perl100 4 Helga Joens
Perl100 5 Maggie Jomomma
Perl100 6 Mary Meigh
Perl100 7 JONES
Perl100 8 Bob JONES
Perl100 9 Ted Applebee
Perl100 10 Jon Nesbitt
Perl100 11 Mary Lamacker
Perl100 12 Mark Jackson
Count open seats by class

SELECT
COURSE ,
COUNT(SEAT_NUM) "# Open Seats"
FROM VW_OCCUPIED_SEATS_BY_CLASS
GROUP BY COURSE

/

COURSE # Open Seats
----------------------------
dbOrchestra100 16
Perl100 12
Column name considerations
The column name MUST be unique in a view. Note the following example.

CREATE OR REPLACE View vw_NAME_CONFLICT
AS
SELECT
a.CLASSES_NUM ,
b.CLASSES_NUM

FROM CLASSES a
JOIN CLASSESREGISTRATION b
ON a.CLASSES_NUM = b.CLASSES_NUM

/

Duplicate column name 'classes_num'

Here is how to resolve this issue. Create a unique name using "as".

CREATE OR REPLACE VIEW vw_NAME_CONFLICT
AS
SELECT a.CLASSES_NUM "CLASSES_CLASSES_NUM" ,
b.CLASSES_NUM "CLASSREGISTRATION_CLASSES_NUM"

FROM CLASSES a
JOIN CLASSREGISTRATION b
ON a.CLASSES_NUM = b.CLASSES_NUM

/

Drop a View
DROP VIEW COURSEREGISTRATION.VW_NAME_CONFLICT
/

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