MySQL Views.md

Author: zhangyijun402

MySQL Views.md

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+原文地址：https://www.mysqltutorial.org/mysql-views/
+
+
+
+# MySQL Views
+
+**Summary**: in this tutorial, you will learn about MySQL views and how to manipulate views effectively.
+
+## Introduction to MySQL Views
+
+Let’s see the following tables `customers` and `payments` from the [sample database](https://www.mysqltutorial.org/getting-started-with-mysql/mysql-sample-database/).
+
+![image](https://github.com/user-attachments/assets/1a739552-bfa9-4b02-a671-96786238eb86)
+
+This [query](https://www.mysqltutorial.org/mysql-basics/mysql-select-from/) returns data from both tables `customers` and `payments` using the [inner join](https://www.mysqltutorial.org/mysql-basics/mysql-inner-join/):
+
+![image](https://github.com/user-attachments/assets/e6bc3df2-de85-4954-bfb5-b2488f789c9b)
+
+Here is the output:
+
+![image](https://github.com/user-attachments/assets/077c2565-16a3-4a2c-b484-ff21e282b702)
+
+Next time, if you want to get the same information including customer name, check number, payment date, and amount, you need to issue the same query again.
+
+One way to do this is to save the query in a file, either .txt or .sql file so that later you can open and execute it from MySQL Workbench or any other MySQL client tools.
+
+A better way to do this is to save the query in the database server and assign a name to it. This named query is called a **database view,** or simply, **view**.
+
+By definition, a view is a named query stored in the database catalog.
+
+To create a new view you use the `CREATE VIEW` statement. This statement creates a view `customerPayments` based on the above query above:
+
+![image](https://github.com/user-attachments/assets/30fdca68-1c80-4a65-9a1f-7d339512469c)
+
+After you execute the `CREATE VIEW` statement, MySQL creates the view and stores it in the database.
+
+Now, you can reference the view as a table in SQL statements. For example, you can query data from the `customerPayments` view using the `SELECT` statement:
+
+![image](https://github.com/user-attachments/assets/a8ad17e3-4640-4fdc-ba92-9748d1eee145)
+
+As you can see, the syntax is much simpler.
+
+Note that a view does not physically store the data. When you issue the `SELECT` statement against the view, MySQL executes the underlying query specified in the view’s definition and returns the result set. For this reason, sometimes, a view is referred to as a virtual table.
+
+MySQL allows you to create a view based on a `SELECT` statement that retrieves data from one or more tables. This picture illustrates a view based on columns of multiple tables:
+
+![image](https://github.com/user-attachments/assets/4e028eb6-31c4-4b41-a4ec-24d0a06e0106)
+
+In addition, MySQL even allows you to create a view that does not refer to any table. But you will rarely find this kind of view in practice.
+
+For example, you can create a view called daysofweek that return 7 days a week by executing the following query:
+
+![image](https://github.com/user-attachments/assets/de8d29fd-c48b-412e-89c1-596305cc47eb)
+
+You can query data from the daysofweek view as follows:
+
+![image](https://github.com/user-attachments/assets/d3547d87-ac6f-45dc-959b-85f244da458c)
+
+This picture shows the output:
+
+![image](https://github.com/user-attachments/assets/d6d42fa8-8a7a-4133-bc27-4040be5f9f24)
+
+## Advantages of MySQL Views
+
+MySQL views bring the following advantages.
+
+### 1) Simplify complex query
+
+Views help simplify complex queries. If you have any frequently used complex query, you can create a view based on it so that you can reference the view by using a simple `SELECT` statement instead of typing the query all over again.
+
+### 2) Make the business logic consistent
+
+Suppose you have to repeatedly write the same formula in every query. Or you have a query that has complex business logic. To make this logic consistent across queries, you can use a view to store the calculation and hide the complexity.
+
+### 3) Add extra security layers
+
+A table may expose a lot of data including sensitive data such as personal and banking information.
+
+By using views and privileges, you can limit which data users can access by exposing only the necessary data to them.
+
+For example, the table `employees` may contain SSN and address information, which should be accessible by the HR department only.
+
+To expose general information such as first name, last name, and gender to the General Administration (GA) department, you can create a view based on these columns and grant the users of the GA department the view, not the entire table `employees` .
+
+### 4) Enable backward compatibility
+
+In legacy systems, views can enable backward compatibility.
+
+Suppose, you want to normalize a big table into many smaller ones. And you don’t want to impact the current applications that reference the table.
+
+In this case, you can create a view whose name is the same as the table based on the new tables so that all applications can reference the view as if it were a table.
+
+Note that a view and table cannot have the same name so you need to [drop the table](https://www.mysqltutorial.org/mysql-drop-table) first before creating a view whose name is the same as the deleted table.
+
+## Managing views in MySQL
+
+- [Create views](https://www.mysqltutorial.org/mysql-views/mysql-create-view/) – show you how to use the `CREATE VIEW` statement to create a new view in the database.
+- [Understand view processing algorithms](https://www.mysqltutorial.org/mysql-views/mysql-view-processing-algorithms/) – learn how MySQL processes a view.
+- [Create updatable views](https://www.mysqltutorial.org/mysql-views/mysql-updatable-views/) – learn how to create updatable views.
+- Create views with a `WITH CHECK OPTION` – ensure the consistency of views using the `WITH CHECK OPTION` clause.
+- `LOCAL & CASCADED` and `WITH CHECK OPTION` – specify the scope of the check with `LOCAL` and `CASCADED` options.
+- [Show views](https://www.mysqltutorial.org/mysql-views/mysql-show-view/) – provide ways to find views in a database.
+- [Show create view](https://www.mysqltutorial.org/mysql-views/mysql-show-create-view/) – learn how to display the statement that creates a view.
+- [Rename views](https://www.mysqltutorial.org/mysql-views/mysql-rename-view/) – change the name of a view to another.
+- [Drop views](https://www.mysqltutorial.org/mysql-views/mysql-drop-view/) – guide you on how to remove one or more existing views.