SQL中的視圖是一個保存的查詢，可像表一樣重複使用，但不存儲數(shù)據(jù)本身。它在運行時顯示來自一個或多個表的數(shù)據(jù)，如同數(shù)據(jù)庫的一個窗口。視圖的主要用途包括：1. 簡化複雜查詢——可將長且重複的查詢保存為視圖以便後續(xù)調用；2. 提高安全性——通過視圖限制用戶訪問特定列或行；3. 保持一致性——確保多人獲取相同的過濾或計算結果。創(chuàng)建視圖使用CREATE VIEW語句，例如創(chuàng)建顯示高價產品的視圖：CREATE VIEW expensive_products AS SELECT product_name, price FROM products WHERE price > 1000; 創(chuàng)建後可像普通表一樣查詢。需注意視圖不存儲數(shù)據(jù)，因此每次執(zhí)行時都會從源表獲取最新信息。使用視圖時也需留意潛在問題：1. 性能影響——複雜查詢可能拖慢執(zhí)行速度；2. 依賴關係——底層表結構變更可能導致視圖出錯；3. 更新限制——部分數(shù)據(jù)庫不支持通過視圖修改數(shù)據(jù)，尤其是涉及連接或聚合的情況；此外應避免命名衝突。

A view in SQL is like a saved query that you can reuse as if it were a table. It doesn't store data itself — instead, it shows data from one or more tables when you run it. Think of it as a window into your database that gives you a specific look at the data without duplicating anything.

Why Use Views?

Views are handy for a few main reasons:

• Simplify complex queries — You can save long, repetitive queries as a view and call them by name later.
• Improve security — You can let users access only certain columns or rows through a view, without giving them full access to the underlying tables.
• Keep things consistent — If multiple people need the same filtered or calculated data, a view ensures everyone gets the same results the same way.

For example, if you often need to see a list of active customers with their total orders, you can create a view for that instead of rewriting the JOIN and WHERE clauses every time.

How to Create a View

Creating a view uses the CREATE VIEW statement. The basic syntax looks like this:

 CREATE VIEW view_name AS
SELECT column1, column2, ...
FROM table_name
WHERE condition;

Let's say you want a view showing expensive products:

 CREATE VIEW expensive_products AS
SELECT product_name, price
FROM products
WHERE price > 1000;

Once created, you can query it like a regular table:

 SELECT * FROM expensive_products;

Just remember: since views don't store data, they always pull the latest info from the source tables when you use them.

Things to Watch Out For

Even though views are useful, there are some common gotchas:

• Performance impact — If your view runs a heavy query, it might slow things down every time you use it.
• Dependencies — If the underlying table changes structure, your view might break.
• Not always updatable — Some databases let you update data through a view, but many restrict it, especially if the view uses joins or aggregations.

Also, be careful with naming — make sure your view names don't clash with existing tables or other views.

基本上就這些。

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