SQL CREATE VIEW 語句

什么是視圖？

在 SQL 中，視圖是基于 SQL 語句的結(jié)果集的可視化的表。

視圖包含行和列，就像一個真實的表。視圖中的字段就是來自一個或多個數(shù)據(jù)庫教程中的真實的表中的字段。我們可以向視圖添加 SQL 函數(shù)、WHERE 以及 JOIN 語句，我們也可以提交數(shù)據(jù)，就像這些來自于某個單一的表。

注釋：數(shù)據(jù)庫的設(shè)計和結(jié)構(gòu)不會受到視圖中的函數(shù)、where 或 join 語句的影響。

SQL CREATE VIEW 語法

CREATE VIEW view_name AS

SELECT column_name(s)

FROM table_name

WHERE condition

注釋：視圖總是顯示最近的數(shù)據(jù)。每當(dāng)用戶查詢視圖時，數(shù)據(jù)庫引擎通過使用 SQL 語句來重建數(shù)據(jù)。

SQL CREATE VIEW 實例

可以從某個查詢內(nèi)部、某個存儲過程內(nèi)部，或者從另一個視圖內(nèi)部來使用視圖。通過向視圖添加函數(shù)、join 等等，我們可以向用戶精確地提交我們希望提交的數(shù)據(jù)。

樣本數(shù)據(jù)庫 Northwind 擁有一些被默認安裝的視圖。視圖 "Current Product List" 會從 Products 表列出所有正在使用的產(chǎn)品。這個視圖使用下列 SQL 創(chuàng)建：

CREATE VIEW [Current Product List] AS

SELECT ProductID,ProductName

FROM Products

WHERE Discontinued=No我們可以查詢上面這個視圖：

SELECT * FROM [Current Product List]Northwind 樣本數(shù)據(jù)庫的另一個視圖會選取 Products 表中所有單位價格高于平均單位價格的產(chǎn)品：

CREATE VIEW [Products Above Average Price] AS

SELECT ProductName,UnitPrice

FROM Products

WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)

我們可以像這樣查詢上面這個視圖：

SELECT * FROM [Products Above Average Price]另一個來自 Northwind 數(shù)據(jù)庫的視圖實例會計算在 1997 年每個種類的銷售總數(shù)。請注意，這個視圖會從另一個名為 "Product Sales for 1997" 的視圖那里選取數(shù)據(jù)：

CREATE VIEW [Category Sales For 1997] AS

SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales

FROM [Product Sales for 1997]

GROUP BY CategoryName

我們可以像這樣查詢上面這個視圖：

SELECT * FROM [Category Sales For 1997]我們也可以向查詢添加條件?，F(xiàn)在，我們僅僅需要查看 "Beverages" 類的全部銷量：

SELECT * FROM [Category Sales For 1997]

WHERE CategoryName='Beverages'

SQL 更新視圖

您可以使用下面的語法來更新視圖：

SQL CREATE OR REPLACE VIEW Syntax

CREATE OR REPLACE VIEW view_name AS

SELECT column_name(s)

FROM table_name

WHERE condition

現(xiàn)在，我們希望向 "Current Product List" 視圖添加 "Category" 列。我們將通過下列 SQL 更新視圖：

CREATE VIEW [Current Product List] AS

SELECT ProductID,ProductName,Category

FROM Products

WHERE Discontinued=No

SQL 撤銷視圖

您可以通過 DROP VIEW 命令來刪除視圖。

SQL DROP VIEW Syntax

DROP VIEW view_name

OR REPLACE VIEW `<your_view_name>`

下面為老外網(wǎng)站上的教程

your_view_name>`

AS

...其次是正常的SQL的SELECT。這個SELECT可以包含一個WHERE子句或其他需要，可以對SELECT語句放在別的事情。該方案是無止境的。這實際上取決于視圖的目的。

正如你可以看到在我們看來，我們正在格式化的姓氏和名字。這是一個很常見的事做有一個觀點，即我們已經(jīng)這樣做了保存有寫在每一個查詢的WHERE這是一個要求的功能。你也可以看到，我們已經(jīng)采取的出生日期列和計算年齡。

執(zhí)行視圖

執(zhí)行一個SQL視圖

下面的例子顯示所有從視圖代碼。你也可以做一個SELECT*，或進一步限制列你想看到的。您還可以添加額外的行限制的看法，因為我們的做法。

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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