MySQL foreign keys can be set as primary keys, but are generally not recommended. The reasons are as follows: Foreign keys assume the responsibility of maintaining relationships, and the responsibilities are too heavy after setting them as primary keys. Redundant data increases maintenance costs. Foreign keys depend on the primary key of another table and may cause inconsistencies when modified.
Can MySQL foreign key be a primary key? The answer is: Yes, but it is not usually recommended.
This question seems simple, but it has hidden mystery. On the surface, aren’t foreign keys used to associate tables? Isn't the primary key used to uniquely identify the record? It seems that there is nothing wrong with setting the foreign key as the primary key. But in practical applications, doing this often makes you dig a hole for yourself.
Let's first review the concepts of primary and foreign keys in MySQL. The primary key, as the name suggests, is a column in the table that uniquely identifies each record. It ensures the uniqueness of the data and does not allow duplicate values. A foreign key is used to establish a relationship between tables. It refers to the primary key of another table to ensure the consistency and integrity of the data.
By understanding these, we can understand why it is not usually recommended to set foreign keys as primary keys. The reason is simple: the primary key should focus on identifying the uniqueness of the records in this table, and the responsibility of the foreign key is to maintain the relationship with other tables. Setting a foreign key as a primary key means that you force the foreign key to assume dual responsibilities, which is like having an employee take charge of two completely different departments at the same time, which is inefficient and prone to errors.
Imagine if your foreign key is the primary key of another table, then you are actually copying the primary key of another table into this table. This not only adds redundant data, but can also lead to data inconsistencies. If the primary key of another table changes, the data in your table also needs to be updated accordingly, otherwise data will be inconsistent. This is not a minor problem, especially when the data volume is large, it can be very troublesome to maintain and even cause unpredictable errors.
Of course, in special cases, you may need to do this. For example, you have a table that is specifically used to store certain public information of other tables, and the primary key of this table is also a unique identifier of other tables. In this case, it is feasible to set the foreign key as the primary key, but be careful and fully consider the integrity and consistency of the data.
Let's look at an example, suppose there are two tables: users and orders . The users table has the primary key user_id , and the orders table has the foreign key user_id , which refers to the primary key of users table.
<code class="sql">-- users 表CREATE TABLE users ( user_id INT PRIMARY KEY, username VARCHAR(255) ); -- orders 表CREATE TABLE orders ( order_id INT PRIMARY KEY, user_id INT, order_date DATE, FOREIGN KEY (user_id) REFERENCES users(user_id) );</code>
In this example, user_id of orders table is a foreign key, but it is not a primary key. If we force user_id to be the primary key, then there cannot be two orders in orders table belong to the same user, which obviously does not conform to the actual situation.
All in all, while MySQL allows you to set foreign keys as primary keys, this is usually not a best practice. Unless you have a very deep understanding of database design and have good reasons, it is best to avoid doing so. Remember, clear database design can ensure the stability and maintainability of the system. Don't sacrifice the robustness of the system in pursuit of so-called simplicity. It’s like building a house. Only when the foundation is laid firmly can a high-rise building be built. Otherwise, no matter how beautiful the decoration is, it cannot cover up the risk of the foundation being unstable.
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