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SQL QuizSQL Views
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A view is a virtual table.
This chapter shows how to create, update, and delete a view.
SQL CREATE VIEW Statement
In SQL, a view is a virtual table based on the result-set of an SQL statement.
A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database.
You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from one single table.
SQL CREATE VIEW Syntax
| CREATE VIEW view_name AS SELECT column_name(s) FROM table_name WHERE condition |
Note: A view always shows up-to-date data! The database engine recreates the data, using the view's SQL statement, every time a user queries a view.
SQL CREATE VIEW Examples
If you have the Northwind database you can see that it has several views installed by default.
The view "Current Product List" lists all active products (products that are not discontinued) from the "Products" table. The view is created with the following SQL:
| CREATE VIEW [Current Product List] AS SELECT ProductID,ProductName FROM Products WHERE Discontinued=No |
We can query the view above as follows:
| SELECT * FROM [Current Product List] |
Another view in the Northwind sample database selects every product in the "Products" table with a unit price higher than the average unit price:
| CREATE VIEW [Products Above Average Price] AS SELECT ProductName,UnitPrice FROM Products WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products) |
We can query the view above as follows:
| SELECT * FROM [Products Above Average Price] |
Another view in the Northwind database calculates the total sale for each category in 1997. Note that this view selects its data from another view called "Product Sales for 1997":
| CREATE VIEW [Category Sales For 1997] AS SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales FROM [Product Sales for 1997] GROUP BY CategoryName |
We can query the view above as follows:
| SELECT * FROM [Category Sales For 1997] |
We can also add a condition to the query. Now we want to see the total sale only for the category "Beverages":
| SELECT * FROM [Category Sales For 1997] WHERE CategoryName='Beverages' |
SQL Updating a View
You can update a view by using the following syntax:
SQL CREATE OR REPLACE VIEW Syntax
| CREATE OR REPLACE VIEW view_name AS SELECT column_name(s) FROM table_name WHERE condition |
Now we want to add the "Category" column to the "Current Product List" view. We will update the view with the following SQL:
| CREATE VIEW [Current Product List] AS SELECT ProductID,ProductName,Category FROM Products WHERE Discontinued=No |
SQL Dropping a View
You can delete a view with the DROP VIEW command.
SQL DROP VIEW Syntax
| DROP VIEW view_name |
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