How to add computed columns in SQL?

How to add computed columns in SQL? Temporary calculation: Using the SELECT statement, there is no need to modify the table structure, and the calculation results only exist in the query results. Permanent save: Add new columns to the table, use the UPDATE statement to fill the data, and the calculation results are permanently saved in the table, but are not automatically updated. Usage views: Create virtual tables, encapsulate calculation results, easy to use, and does not occupy storage space.

SQL adds a computed column? Listen to me to tell you in detail

You ask how to add a calculated column in SQL? This question is very easy, but it actually has a secret. Many beginners think that is just adding a field and then calculating the calculation? naive! It depends on what your goal is, whether it is temporary calculation or permanent storage? This determines your approach.

Let’s talk about the basics first, you have to understand that the “columns” in SQL are not as casual as Excel tables. It is related to the table structure, and you must be cautious when changing it. Temporary calculations can be done with SELECT statements, and there is no need to change the table structure at all. For example, if you want to calculate the total price of each order, the order table has unit price and quantity, directly:

 <code class="sql">SELECT order_id, price * quantity AS total_price FROM orders;</code>

This AS total_price gives the calculation result a name. The total_price column only exists in the query result this time, and the table itself has not changed. It's like a magic trick. It's like a trick, but it's gone in a blink of an eye. Convenient and fast, but the data is not lasting.

If you want to save the calculation results permanently, you have to be serious. You have to add a new column to the table and then use the UPDATE statement to fill in the data. For example, add a total_price column to the orders table:

 <code class="sql">ALTER TABLE orders ADD COLUMN total_price DECIMAL(10, 2); -- 数据类型要选对！</code>

Then update the data:

 <code class="sql">UPDATE orders SET total_price = price * quantity;</code>

This time total_price is added to the table, so it will be convenient to query in the future, and you don’t have to calculate it every time. but! Note that this is just a static snapshot. The unit price or quantity will change in the future, total_price will not be automatically updated. You have to regularly maintain it with UPDATE statements, or consider triggers to update it automatically. This is an advanced topic and depends on the support level of your database system.

There is a pit here, which is the selection of data types. DECIMAL(10, 2) is a random choice. You have to choose the appropriate data type according to the actual situation, otherwise it may overflow or the accuracy may be insufficient, resulting in an incorrect calculation result. This is not a joke, the data is wrong, and the consequences are very serious.

There is another more advanced way to play, which is to use views. A view can wrap the calculation results into a virtual table, which is like a real one, but it does not take up actual storage space. for example:

 <code class="sql">CREATE VIEW order_with_total AS SELECT order_id, price, quantity, price * quantity AS total_price FROM orders;</code>

In the future, just use the order_with_total view to query, which is convenient and easy to save time. This method combines the advantages of temporary calculation and permanent storage, which is convenient for querying and does not increase the burden of table structure.

In short, there are many ways to add calculation columns in SQL, and which one should be chosen according to your actual needs. Don't be confused by the simplicity on the surface. Only by deeply understanding the concepts of data types, triggers, and views can you write efficient and reliable SQL code. Remember, the code is written for people to read and for machines to execute. Clear and efficient is the king. Only by practicing and thinking more can you become a true SQL master.

The above is the detailed content of How to add computed columns in SQL?. For more information, please follow other related articles on the PHP Chinese website!