Views are virtual tables based on query results in MySQL, used to simplify complex queries, improve security and unified data access methods. Its creation syntax is CREATE VIEW view_name AS SELECT statement, for example, you can create a completed_orders view to filter completed orders. To modify the view, you can use CREATE OR REPLACE VIEW or ALTER VIEW, and to delete it, you can use DROP VIEW. When using it, you need to pay attention to: the view name is unique, does not store actual data, cannot include TEMPORARY tables, updates are restricted, and performance depends on the original query efficiency.
Creating a view (View) is not complicated in MySQL. It is essentially saving a commonly used query statement into a "virtual table", and you can use it directly like a table lookup in the future. The key is to write SQL queries well and understand the limitations and uses of views.
What is a view? Why use it?
A view is a virtual table based on SQL query results. It does not store actual data, but dynamically obtains data from the underlying table.
Common uses include:
- Simplify complex queries, such as multi-table joins, aggregate functions, etc.
- Improve security, hide sensitive fields or structures
- Unified data access methods, allowing different users to see different data subsets
For example: You often need to view the order amount sum and customer information. It is troublesome to write JOIN and SUM every time, so you can create a view to simplify this operation.
How to create a basic view?
The syntax is simple:
CREATE VIEW view_name AS SELECT column1, column2, ... FROM table_name WHERE condition;
For example, suppose there is an orders table and you want to create a view that only displays completed orders:
CREATE VIEW completed_orders AS SELECT order_id, customer_id, amount FROM orders WHERE status = 'completed';
Then you can query it like a normal table:
SELECT * FROM completed_orders;
Some points to note:
- The view name must be unique and cannot be duplicated with existing tables or views.
- The SELECT statement in the view definition can contain JOIN, GROUP BY, subquery, etc.
- ORDER BY is not recommended in view unless it is matched with LIMIT
How to modify and delete views?
If you find that the view is written incorrectly, you can modify it or delete it directly and rebuild it.
Modify the view:
CREATE OR REPLACE VIEW view_name AS SELECT new_columns...
or:
ALTER VIEW view_name AS SELECT new_columns...
Delete the view:
DROP VIEW view_name;
Tip: If you are not sure whether the view exists, you can use:
DROP VIEW IF EXISTS view_name;
This avoids error reporting.
What are the restrictions when using views?
Although the view is convenient, it is not omnipotent. Some situations need to be paid attention to:
- There cannot be a TEMPORARY table in the view
- If the view refers to multiple tables, the data cannot be updated directly through the view (INSERT, UPDATE, DELETE)
- In terms of performance, the view is actually a "real-time execution" query. If the original query is slow, the view will also be slow.
In addition, be careful when managing permissions to ensure that only authorized users can access sensitive views.
Basically that's it. The key to writing a good view is to first write out clear and efficient SQL queries, and then encapsulate them. It seems simple, but once you get used to it, it will save a lot of duplicate code.
The above is the detailed content of how to create a view in mysql. For more information, please follow other related articles on the PHP Chinese website!
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